The human CDC25 tyrosine phosphatases trigger activation of CDC2 by removing inhibitory phosphate from tyrosine and threonine residues of the cyclin-dependent kinases.The genes encoding these phosphatases are suspected of being potential oncogenes because of their role in promoting cell division. Three human CDC25 genes have been identified: CDC25A, CDC25B , and CDC25C . The CDC25A gene is mapped to 3p21 .in rodent cells, human CDC25A or CDC25B but not CDC25C phosphatases cooperate with either the gly12-to-val mutation of the HRAS gene or loss of RB1 in oncogenic focus formation.