STK31 (Serine/threonine protein kinase 31) is similar to a mouse gene that encodes a putative protein kinase with a tudor domain, and shows testis specific expression. Alternative splicing results in multiple transcript variants encoding different isoforms. Protein kinases are enzymes that transfer a phosphate group from a phosphate donor, generally the g phosphate of ATP, onto an acceptor amino acid in a substrate protein. By this basic mechanism, protein kinases mediate most of the signal transduction in eukaryotic cells, regulating cellular metabolism, transcription, cell cycle progression, cytoskeletal rearrangement and cell movement, apoptosis, and differentiation. With more than 500 gene products, the protein kinase family is one of the largest families of proteins in eukaryotes.