Marc Wallace Kirschner (born February 28, 1945) is an American cell biologist and biochemist and the founding chair of the Department of Systems Biology at Harvard Medical School.

He participated in the NSF Graduate Research Fellowship Program at the National Science Foundation in 1966, and earned a doctorate in biochemistry from the University of California, Berkeley in 1971.

In studies at UC San Francisco of the frog embryo as a model system of cell development, Kirschner identified the first inducer of embryonic differentiation, fibroblast growth factor (FGF),[11] an early finding in the field of signal transduction.

Because of dynamic instability, some individual microtubules that are not stabilized are at risk of collapse (or “catastrophe” as Kirschner named it), allowing re-use of the tubulin monomers.

This recognition of self-organization in biological systems has been highly influential, and helped shape the view of the cytoplasm as a collection of dynamic molecular machines.

[27] In the most recent book, Kirschner and Gerhart proposed a new theory of "facilitated variation" that aims to answer the question: How can small, random genetic changes be converted into useful changes in complex body parts?

[31] Kirschner helped launch the monthly, peer-reviewed journal PLoS Biology in October 2003 as a member of the editorial board and senior author of a paper in the inaugural issue.

The journal was the first publishing venture from the San Francisco-based Public Library of Science (PLoS), which had begun three years previously as a grassroots organization of scientists advocating free and unrestricted access to the scientific literature[32]