Small cell lung cancer (SCLC) subtype classification, based on high-level expression of key transcriptional regulators; ASCL1 (SCLC-A), NEUROD1 (SCLC-N), POU2F3 (SCLC-P), and YAP1 (SCLC-Y), has recently been proposed. YAP1 (and POU2F3) has attracted attention as an important factor for non-neuroendocrine (non-NE) phenotypic subtyping of SCLC. However, subsequent studies reported that YAP1 expression alone cannot define a single group in primary SCLC, which makes it difficult to understand what SCLC-Y is by focusing only on SCLC. In this review, we concluded that YAP1 is an essential anti-neuroendocrine factor in both SCLC and non-small cell lung cancer (NSCLC) based on previous studies, including our own analysis of the cell lines and primary tumors of SCLC and NSCLC. The classification of SCLC-Y is a concept mainly established from the analysis of cell lines, and SCLC-Y cell lines correspond to “variant type” SCLC cell lines. Primary SCLC and large cell neuroendocrine carcinoma (LCNEC) are typically heterogeneous tumors composed mostly of NE-type cells, but they contain a small number of non-NE-type cells. Importantly, individual cells with NE features exhibit YAP1 loss, whereas the non-NE-type cells exhibit YAP1 expression. Although rare in primary SCLC, some cases of primary LCNEC have many YAP1-positive cells, which is correlated with chemotherapy resistance. YAP1 staining may be useful in the exclusion diagnosis of SCLC or in the selection of treatment for LCNEC.