Cyclins are eukaryotic proteins that play an active role in controlling cell division cycles and regulate cyclin dependent kinases (CDKs). In this study, we identified and characterized a new member of the human cyclin family. Its cDNA sequence contains an open reading frame of 864 bp, encoding a protein of 287 amino acids. This putative cyclin, designated cyclin X, contains a typical cyclin box. Cyclin X shows the highest sequence similarity in cyclin box to cyclin B3 and the similarity between these two proteins also extends to the whole molecule. The expression pattern of cyclin X showed that it was highly expressed in testis. Cyclin X protein was located in nucleus of the H1299 cells and Overexpression of cyclin X activated the transcriptional activities of c-Myc. These results indicate that cyclin X is a novel member of the human cyclin family and positive regulator of c-Myc-mediated gene transcription.