MDM2 and osteosarcoma: Statistically significant gene expression differences between resections and biopsies existed for the expression levels of CDKN1A, MDM2, BCL2L1, PTEN, and WWOX. CDKN1A is activated in response to activation of the ATM/TP53 DNA damage checkpoint that accommodates double-stranded DNA repair and inhibits cell cycle progression by CDK4 [54], MDM2 is an inhibitor of TP53 [55], BCL2L1 is an anti-apoptotic factor [56], PTEN is a tumor suppressor commonly lost in osteosarcomas [23], and WWOX is a tumor suppressor that inhibits RUNX2 activity [24].