In parallel, other studies focused on neuroendocrine carcinomas and high-grade neuroendocrine tumors of specific sites to verify whether specific “site-markers” could help to distinguish them from MCC (e.g., CDX2 for gastrointestinal tumors, GATA3 for urogenital and breast tumors, and NKX3.1 for prostate tumors) [106,107,108,109]. This evidence concerns the gene CDX2 and breast neoplasm.