[1][5][6] He obtained a doctoral degree in cell and development biology from the University of Medicine and Dentistry of New Jersey's Robert Wood Johnson Medical School in 1996.

[2][7] Krein also volunteers with Faces of Honor, where he performs reconstructive surgery for victims of domestic violence and veterans of the War in Afghanistan and the Iraqi conflict.

[7] Krein has been on faculty at Thomas Jefferson University since 2007,[1][2] and has served as the Senior Director of Health Policy and Innovation at the Sidney Kimmel Cancer Center since 2016.

[19] Krein's role with the Biden campaign while working as an executive of a health technology firm led to speculation about a potential conflict of interest.

[19] Krein stated that he does not have a formal role in the Biden campaign, but has participated in briefings on the coronavirus based on his experience treating patients and coordinating Thomas Jefferson University Hospital's response to the outbreak.

[3] They married in June 2012 in a Catholic-Jewish interfaith ceremony at St. Joseph's on the Brandywine in Greenville, Delaware,[3] officiated by a Catholic priest assisted by a rabbi.