Cyclins constitute a growing family of regulatory proteins that complex with, and activate, protein kinases involved in cell cycle control. Dysregulation of cyclin expression and/or cyclin-dependent kinase (cdk) activities may play a pivotal role in oncogenesis. In this report, we characterize a novel human cyclin gene by molecular cloning. This gene, designated CYCG1, encodes a human homologue of the rat G-type cyclin, exhibiting structural features and conserved sequence motifs of identified G(1) cyclins. The CYCG1 gene is expressed constitutively in synchronized human WI-38 fibroblasts and MG-63 osteosarcoma cells, which is reminiscent of CLN3 in Saccharomyces cerevisiae. Marked overexpression of CYCG1 is observed in a subset of human osteosarcoma cells, providing a potential link to cancer.