The objective is to study the genetic basis of the Gl period of the mammalian cell cycle. An understanding of Gl is important because this is the period in the life of a cell during which control of cellular proliferation is exercised. The nature of this regulation is relevant to uncontrolled cell proliferation characteristic of neoplasia. I plan to examine using somatic cell genetic techniques the difference(s) between cells that lack altogether a Gl period (Gl-less or Gl-cells) and those that express Gl (Gl ion cells), testing among other things the dominance vs. recessiveness of the Gl-less phenotype in cell hybrids. Gl ion mutants of a Gl-cell with be isolated and characterized. These mutants will provide genetic tools for inquiring into 1) the genetic complexity of the Gl- yields Gl ion phenotypic change; 2) the role of such cellular processes as cell growth and centriole duplication in progression of the cell cycle, and eventually 3) the molecular nature of events associated with initiation of DNA synthesis. Another aspect of this investigation is to ask whether cells in vivo all share the same genetic basis for the expression of their Gl periods or whether the cells of different tissues posses different genetic bases for their Gl period, i.e. differently based Gl periods might be correlated with the differentiation fate of the cell in development.