Board of Directors

Taejoon Ahn, M.D. became the president of John Muir Medical Group (JMMG) in January of 2014. He oversees a medical group of over 300 clinicians, mostly in primary care. JMMG is the exclusive contracted provider of primary care services for John Muir Health, partnering closely for over 20 years. Prior to the presidency, Dr. Ahn served as vice president and medical director of urgent care services for JMMG. He also worked as faculty for the University of California, San Francisco's National HIV/AIDS Clinicians' Consultation Center. He is currently an Assistant Clinical Professor of Family & Community Medicine for UCSF.

Dr. Ahn also serves as director on the board of John Muir Health as well as the board of Canopy Health. Canopy Health is a joint venture between multiple provider groups to offer a large network in the San Francisco Bay Region.

Dr. Ahn received his medical degree from the University of California San Francisco, where he also completed a residency in Family & Community Medicine. He received his Masters in Public Health with an emphasis in Epidemiology from UC Berkeley. He is an alumnus of the California Healthcare Foundation's Leadership Fellowship and is a certified physician executive with the American Association for Physician Leadership (formerly the American College of Physician Executives).

Dr. Tanya Threewitt has been working in Concord with John Muir Medical Group since 2002, splitting her time between her practice and administrative responsibilities to the group.  She currently is the Ambulatory Medical Director of Operations and the daily operations of the clinics in JMMG. Dr. Threewitt has also been heavily involved in Epic, John Muir Medical Groups Electronic Health Record (EHR).  In her spare time, she likes to read and travel.

Born and raised in San Jose, California, Dr. Threewitt received an undergraduate degree in psychology from UC Davis.  She graduated from Hahnemann Medical School in Philadelphia and completed her Family Medicine residency through UC Davis in Modesto, California at the Stanislaus County Doctor’s Medical Center. 

Dr. Nicole Jones (Niki) started her career as a hospitalist straight out of an Internal Medicine Residency at California Pacific Medical Center in San Francisco in the fall of 2003.  Originally part of the independent California Hospital Physicians Medical Group, Inc., as the result of a merger, she became a member of John Muir Medical Group in 2005.

In addition to her clinical work as a hospitalist, Dr. Jones has held various leadership positions, including Associate Medical Director of the John Muir Medical Group Hospitalists at the Walnut Creek Medical Center as well as the Chair of the Hospitalist Division. Dr. Jones was elected to the John Muir Medical Group Executive Board in January of 2015, and became Secretary in January, 2017.

Since September 2015, Dr. Jones has divided her time between her clinical work as a hospitalist and her newer role as Cross Campus Medical Director of Inpatient Case Management for John Muir Health. In January of 2016, she assumed the role of Associate Medical Director to help launch the John Muir Hospitalist Program at San Ramon Regional Medical Center.

Born & raised in Half Moon Bay, California, Niki earned a Bachelor's of Science in Biochemistry at University of California, Davis in 1993. She completed osteopathic school at the College of Osteopathic Medicine of the Pacific in Pomona, California in 2003. When not working, she enjoys spending time with her husband Geoff and daughters Sofie and Olivia.

Dr. Cortez joined John Muir Medical Group in 2000 and currently serves on the Board of Directors.  Graduating from the University of California with a Bachelor of Science in Chemical Engineering, he began his career with Pacific Gas and Electric Company as an energy analyst.  He was the recipient of two Ideas in Action awards and Speaker of the Year in the Toastmaster Club.  During this time, he volunteered for the United Way, eventually chairing the Diablo Division and was a mentor for Junior Achievement, in Oakland, CA.  He went on to earn a Master of Business Administration, Finance from California State University, Hayward.   

Community involvement as a hospital volunteer in Oncology Services at John Muir Medical Center in Walnut Creek and as a Bereavement Volunteer at Hospice of Contra Costa led to a career change and in 1997 he received his Doctor of Medicine from the University of California, Irvine and completed his Pediatric Residency at the University of California, San Francisco-Fresno.  During his schooling he was honored as the recipient of The Carl Jefferson Scholarship for academic excellence and community service in 1992 and the Edward Varni Scholarship for academic achievement and community service in 1994 and 1995.  In 2000, he was honored for his humanitarian work with orphans in Nicaragua.

In this spare time, Dr. Cortez enjoys family get-togethers, barbecues and entertaining. 

Bob Kadas, MD is a Pediatrician and has been practicing in Alamo since 1971. He is one of the founding physicians of the John Muir Medical Group and serves on the board of directors. He is also serves at the Associate Medical Director for the John Muir Health/Stanford Children’s Health Pediatric Partnership which is involved in providing expanded pediatric specialty services to our community. In addition, he is involved in continuing education and serves on the board of Symposia Medicus, a non-profit organization providing education to physicians and nurses.

Dr. Kadas received his undergraduate education at University of California at Los Angeles and then crossed town to attend medical school at the University of Southern California. He did his specialty training in pediatrics at the USC/LA County Medical Center. He served for 2 year in the US Navy prior to entering private practice in Alamo. Even after 40+ years, Dr. Kadas continues to enjoy being a pediatrician as well as spending time with his family, playing golf, and even going down the ski slopes a few times a year.

Dr. Obrinsky has been practicing Pediatrics in the Tri-Valley community since 1989.  He attended the School of Public Health at the University of California, Berkeley and obtained a M.S. in Epidemiology in 1982. He then attended medical school at Hahnemann University in Philadelphia and did his residency at Children’s Hospital Oakland.  He is board certified by both the American Board of Pediatrics and the National Board of Physicians and Surgeons.  Upon completing his residency in 1989 he joined San Ramon Valley Primary Care.  Dr. Obrinsky joined the John Muir Medical Group in 2016 when SRVPC was acquired by John Muir Health.  In 2018, Dr. Obrinsky was elected to the Board of Directors for John Muir Medical Group.

Dr. Obrinsky is married and with his wife Kristen has raised 3 sons.  He enjoys playing golf, cooking, traveling, and spending time with family and friends.

Dr. Michael Schierman has been working in the San Ramon/Danville area for over 25 years.  He and his wife, Suedale, moved to California in 1992 from Calgary, Alberta after completing his Family Medicine Residency at Foothills Hospital, University of Calgary.  Dr. Schierman attended the University of Calgary to complete both his undergraduate degree in Psychology and Medical degree, then completed an Internship year at McGill University in Montreal, Quebec.

Dr. Schierman was initially recruited to practice Family Medicine by the San Ramon Medical Center in 1992.  He has served in several leadership roles with this institution including Vice Chief of Staff, Secretary Treasurer and Head of Family Medicine.  Early in his career, as a member of Hill Physicians, he participated in committee work.  As time passed, he became an exclusive member of the Muir Medical Group IPA and served on their Executive Committee as the Vice President and briefly as interim President.  Throughout his career with the IPA, he served on a number of IPA and John Muir Health Network committees and represented the IPA on the Network Board.

Dr. Schierman joined John Muir Medical Group in 2012 and is currently the Medical Director for Recruitment and serves on the Compensation committee.  Dr. Schierman is also in full-time Family Practice in Danville, CA working with Dr. Christine Chang and Dr. Priya Swamy.

Outside of medicine, he enjoys being a loving husband to Suedale, his wife of 32 years, and also being a great dad to kids, Matt, 20 and Michaela, 18.  Musically inclined, Dr. Schierman, plays guitar and enjoys helping Suedale with their church music program.  He enjoys getting together with friends, working out and travelling.  You may see Dr. Schierman in the Danville July 4^th parade as a "semi-professional" Elvis impersonator.