Members of the protein phosphatase 2C (PP2C) family of isoenzymes are serine/threonine protein phosphatases that act as monomers, are magnesium or manganese dependent, and are structurally distinct from PP1, PP2A, and PP2B protein phosphatases. There are 14 mammalian PP2C genes that encode for members of the PP2C family of isoenzymes. Members of this family include PP2C alpha (PPM1A), PP2C beta (PPM1B), PP2C gamma (PPM1G), and PP2C delta (PPM1D). PPM1D has been demonstrated to negatively regulate cell growth. PPM1D dephosphorylates RSK (p90 ribosomal S6 kinase) and regulates ChK2 in the DNA damage response PPM1D acts within the MAPK pathway and can inactivate p38 MAPK which leads to subsequent p53 inactivation PPM1D is amplified in breast tumors and has been demonstrated to enhance steroid hormone action.