tokens: Phosphorylation at Ser 20 ( Ser 23 in mice ) by the Chk2 protein ( coded by the CHEK2 gene ) in response to DNA damage activates p53 by inhibiting binding to , and deactivation by , the MDM2 ( Mouse Minute 2 homolog ; HDM2 ) protein [ 18 ] , [ 19 ] , [ 20 ] .