tokens: Since we were not able to obtain satisfactory technical quality of the kinase assay in cell lines negative for Chk2 ( HCT 15 and HCT 116 ) , we assessed potential background kinase activity due to endogenous Chk2 by performing western blot analysis revealing the endogenous levels of Chk2 in U - 2 - OS cells to be non - significant compared to the exogenously expressed Chk2 levels ( data not shown ) .