Ira Herskowitz (July 14, 1946 – April 28, 2003) was an American phage and yeast geneticist who studied genetic regulatory circuits and mechanisms.

He was particularly noted for his work on mating type switching and cellular differentiation, largely using Saccharomyces cerevisiae as a model organism.

[3] Ira Herskowitz became interested in bacteriophages when he studied with Robert Stuart Edgar at California Institute of Technology.

After completing a degree in 1967, he entered Massachusetts Institute of Technology (MIT), where he studied the molecular biology of the lambda phage.

[4] As a graduate student, Herskowitz made major contributions to the scientific understanding of regulation of gene expression in Lambda phage.

Lambda phage was the first organism whose regulatory strategies became well understood, due in part to Herskowitz' pioneering work.

[2] Saccharomyces cerevisiae became a premier experimental organism for the study of eukaryotic regulation, in part due to Herskowitz's work.

[5] Herskowitz is credited with being the first to use a bar sign in a pathway diagram to denote a negative influence[5] as well as popularizing the phrase "the awesome power of yeast genetics".

Herskowitz identified the mechanism of the mating of yeast in homothallic strains, describing it via the "cassette model" metaphor.

The cell changes types by putting in a different "cassette", replacing the functional section of genetic information with information from one of the silent copies [5][3] Work with Janet Kurjan on models for the pheromone response pathway led to identification of genes for mating pheromones and mating type switching.

[6][11] Herskowitz studied many other areas through the mechanisms of yeast mating, including signal transduction, control of the eukaryotic cell-cycle, RNA transport, the role of chromatin in transcription, meiosis, sporulation, gene expression, and how human genetic variation leads to different responses to drugs.