CDK4 and Ewing sarcoma: Low expression of CDKN2A and/or P53 correlates with decreased apoptosis, or in case of CDKN2A loss also with enhanced self-renewal capacity of MSC.36 INK4A proteins are frequently lost owing to homozygous deletion or p53 mutations, both prominent in patients with ES.37, 47 Enhanced survival in collaboration with decreased pS780-RB expression and increased CDK4 levels could also boost transformation.