DT40 lymphoma B-cells normally express cyclins D1and D2 but not D3. When cyclin D1 expression was extinguished in these cells by gene knockout, specific alterations in their ability to transit the cell cycle were observed. These changes are exemplified by a delay of approximately 2 h in their progression through a normal 14-h cell cycle. This delay results in an increase in the number of cells in the G 2 /M phase population, most likely due to triggering of checkpoints in G 2 /M, inability to enter G 1 normally, and/or alterations of crucial event(s) in early G 1 . The defect(s) in the cell cycle of these D1 "knockout" cells can be rescued by overexpression of any normal mouse D-type cyclin but not by a mutant mouse cyclin D1 protein that lacks the LXCXE motif at its amino terminus. These data suggest that the cell cycle alterations observed in the D1؊/؊ cells are a direct effect of the absence of the cyclin D1 protein and support the hypothesis that the D-type cyclins have separate, but overlapping, functions. Elimination of cyclin D1 also resulted in enhanced sensitivity to radiation, resulting in a significant increase in apoptotic cells. Expression of any normal murine D-type cyclin in the D1؊/؊ cells reversed this phenotype. Intriguingly, expression of the mutant cyclin D1 in the D1 ؊/؊ cells partially restored resistance to radiation-induced apoptosis. Thus, there may be distinct differences in cyclin D1 complexes and/or its target(s) in proliferating and apoptotic DT40 lymphoma B-cells.Regulation of the vertebrate cell cycle requires the periodic formation, activation, and inactivation of unique protein kinase complexes that consist of cyclin (regulatory) and cyclin-dependent kinase (CDK 1 ; catalytic) subunits. Cell cycle-dependent fluctuations in the levels of many of the cyclin proteins are thought to contribute, at least in part, to the activation of these enzymes (1-4). The cyclins are required to regulate many of the p34 cdc2 -related cyclin-dependent protein kinases (CDKs), which have diverse functions prominently linked to the control of cell division (5-16). For example, cyclin B participates in the regulation of the G 2 /M transition by its association with p34 cdc2 , whereas cyclin A appears to be essential for the completion of S phase and entry into G 2 phase in complexes with both p34 cdc2 and cdk2 (17-27). In contrast, complexes formed between the D-type cyclins and both cdk4 and cdk6 integrate growth factor signals and the cell cycle, allowing cells to progress normally through G 1 phase (10, 12, 28 -34). The genetic alterations that occur in this pathway during oncogenesis appear to involve many of its components, including the D-type cyclins, cyclin-dependent protein kinases (cdks), and cyclin-dependent kinase inhibitors (CKIs).Unlike other G 1 cyclins described thus far (e.g. cyclin C and cyclin E), the D-type cyclins are highly homologous to one another and their expression overlaps during G 1 phase, suggesting that this group of cyclins may be somewhat redundant in their function. How...