We have cloned a human cDNA that complements the mutation of ts11, a temperature-sensitive (ts) mutant of the BHK hamster cell line that at the nonpermissive temperature is blocked in progression through the G1 phase of the cell growth cycle. After transfecting human chromosomal DNA into ts11 cells and selecting for cells that had acquired a non-ts phenotype, we screened a genomic library constructed in the EMBL3 lambda vector from a secondary non-ts transformant and isolated a recombinant phage containing human DNA sequences that were uniformly present in primary and secondary non-ts transformants. Genomic probes that recognized an mRNA of about 2 kilobases in human cells were used to isolate from a cDNA expression library two cDNA plasmids that could efficiently transform ts11 cells to a non-ts phenotype. Sequencing of one of these cDNAs revealed a single open reading frame, which could encode a 540 amino acid protein. The ts11 gene has at least two other homologs in human DNA and thus it appears to be part of a small gene/pseudogene family. Experiments with serum-synchronized cells indicate that the expression of the ts11 gene, which is necessary for G1 progression, is itself cell-cycle regulated, being induced in approximately mid-G1.