Leo Sachs (Hebrew: ליאו זקס; ‎14 October 1924 – 12 December 2013) was a German-born Israeli molecular biologist and cancer researcher.

Because there were no animal studies yet at the Institute, Sachs started working on a theory that human amniotic fluid, which bathes the baby in the womb, contains fetal cells that provide information about the fetus.

Using this process, he subsequently discovered and identified a family of proteins that plays a key role in controlling normal blood cell development.

Later named colony stimulating factors (CSF) and interleukins, one of these CSF proteins is now used worldwide in a variety of clinical procedures, including boosting the production of infection-fighting white blood cells in cancer patients undergoing chemotherapy or radiation, and improving the success of bone marrow and peripheral blood cell transplants.

This approach, using retinoic acid combined with chemotherapy, is now standard procedure in treating human promyelocytic leukemia, and it has greatly increased survival rates.