In this study, we have shown that (1) L1CAM mRNA is highly expressed in SCLC cell lines and tissues; (2) its expression correlates with that of NEUROD1 but not ASCL1 mRNA in SCLC cell lines; (3) a newly developed anti-human L1CAM mAb, HSL175, is internalized by L1CAM-positive cells upon binding; and (4) HSL175-DT3C conjugates cause SCLC-N cells to undergo apoptosis in a dose- and time-dependent manner. The gene discussed is ASCL1; the disease is small cell lung carcinoma.