Non-silent mutation of RB1 was similarly more common in SCLC (86 of 110 tumors [78%], P < .001, two-sided Fisher exact test) and SCCB (17 of 19 tumors [89%], P < .001, two-sided Fisher exact test) compared with NECC. Here, RB1 is linked to urinary bladder small cell neuroendocrine carcinoma.