Activation of an S-phase checkpoint in response to ara-C: dysregulation by UCN-01 in ML-1 cells. Exponentially growing ML-1 cells were treated as in Figure 1. (A) pSer³¹⁷Chk1 (top), pSer³⁴⁵Chk1 (middle), and total Chk1 (bottom) in response to ara-C and UCN-01; (B) pTyr¹⁵Cdk2 (top) and total Cdk2 (bottom) in cells depleted for Cdk1 protein. (C) pSer⁴⁷³Akt (top), pSer³⁰⁸Akt (middle), and total Akt (bottom) and (D) p42/44^Erkkinase activity and (E) c-jun kinase activation as measured by endogenous c-jun phosphorylation (top) normalized to the levels of total c-jun (bottom).