The object of the proposed research is to isolate and characterize temperature-sensitive mutants of the G1 (or Go) phase of the cell cycle of human fibroblasts and lymphoblasts in tissue culture. It is hoped that selection of these mutants will yield a model for the regulation and triggering of cell proliferation. The isolated mutants will be studied with particular regard to the capacity of RNA transcription, the synthesis of nonhistone chromosomal protein, and the function of the translational machinery. Using these parameters a sequential mapping and correlation of the mutated functions will be attempted.