Examination of the control of cell type in yeast at the molecular level and understanding of the biochemical basis of the cell-cell interactions involved in the mating process are clearly entering an extremely productive and exciting period. The tools and opportunities are now available to answer fundamental questions with regard to the mechanism of differential gene expression in eukaryotic cells by using cloned a-specific, alpha-specific, and haploid-specific genes as the probes. Basic questions concerning eukaryotic chromosome structure and organization can be addressed by elucidating the properties of the SIR/MAR regulators and their mode of action. Furthermore, the availability both of cloned MAT, HML, and HMR regions and of the HO gene will provide the material for unravelling the enzymology of the DNA transposition that occurs during mating type interconversion. The isolation of the structural genes for the pheromones and mutations that block pheromone production will provide useful information on how extracellular hormones are synthesized, processed, and secreted by eukaryotic cells. Moreover, the apparent mode of action of the phermonones through cyclic AMP as an intracellular "second messenger," and the genetic and biochemical tractability of yeast cells, may allow tracing of the entire pathway of hormonal regulation of a eukaryotic cell division cycle. These and other studies of the developmental biology of yeast cells will provide more important insights into fundamental aspects of the genetic control of developmental processes in eukaryotic cells.