CHEK1 and small cell lung carcinoma: We reasoned that SCLC cells under high replication stress might be characterized by amplification of MYC and its paralogs MYCN and MYCL (30, 31); expression of p-Chk1 (32); sensitivity to inhibitors of cell-cycle checkpoints CHK1 and WEE1 (33); and NE differentiation (12, 29, 34).