| We screened a cDNA library by solid phase phosphorylation and isolated hHR6A as a CDK 2 substrate . hHR6A is the human homologue of the product of the Saccharomyces cerevisiae RAD6 / UBC2 gene , a member of the family of ubiquitin conjugating enzymes . hHR6A is phosphorylated in vitro by CDK 1 and 2 on Ser 120 , a residue conserved in all hHR6A homologues , resulting in a 4 fold increase in its ubiquitin conjugating activity . ^^^ |