tokens: Although no definite conclusion should be drawn from a limited number of observation , the fact that Chk2 not only phosphorylates p53 but also phosphorylates other substrates such as Cdc25A and Cdc25C [ 54 ] and E2F1 in response to etoposide - induced DNA damage [ 61 ] may indicate that inactivation of redundant pathways could take place in parallel .