This report describes the structure of the mRNA, the protein product, and the growth-regulating activity of one of the growth arrest-specific genes, gasl. From the predicted amino acid sequence, in vitro translation of gas7 mRNA, and immunofluorescence of cells In culture, it appears that the gas1 protein is an integral plasma membrane protein whose expression is linked to growth arrest. When gas1 is overexpressed from a constitutive promoter in quiescent cells, the seruminduced transition from the GO to the S phase of the cell cycle is inhibited without affecting the normal early serum response. Ectopic expression of the gas7 gene by microinjedion in normal and transformed NIH 313 cell lines with the notable exception of SV40-transformed 3T3 cells leads to inhibition of DNA synthesis. Thus, gas7 appears to be one component of a negative circuit that governs growth suppression. Its effect is, however, abolished in SV40-transformed cells.