The scope of gene fusions in melanocytic neoplasms is broader than previously recognized, extending well beyond the Spitz-lineage neoplasms where kinase fusions involving ALK, ROS1, NTRK1/2/3, RET, MET, BRAF, and MAP3K8 define biologically and morphologically distinct tumors. Emerging studies demonstrate that a meaningful proportion of conventional non-Spitz lineage melanomas harbor oncogenic fusions. Such fusions may impact clinical behavior, histopathologic presentation and provide opportunities for targeted therapy. The World Health Organization classification of skin tumors, 5th edition, now incorporates fusion status into taxonomy and risk stratification, yet some important questions remain for further investigation: fusion-associated neoplasms can mimic non-melanocytic neoplasm; Spitz-type fusions appear in non-Spitz lesions; and melanocytic differentiation may occur in some other fusion-driven lesions. Broad-panel next-generation sequencing (including RNAseq), together with targeted fluorescence in situ hybridization and immunohistochemistry enhances detection of known and novel fusion partners. Early clinical evidence of TRK, ALK, and ROS1 inhibitor efficacy underscores the translational promise of fusion testing and opens avenues for personalized therapy. This review synthesizes current knowledge on the genomics, histopathology, diagnosis, and therapeutic implications of fusion-driven melanocytic neoplasms, highlighting consensus points and remaining controversies.

Background: Although trichorhinophalangeal syndrome type 1 (TRPS1) was initially thought to be highly sensitive and specific for carcinomas and mesenchymal tumors of mammary origin, more recent data suggest its expression is not limited to breast neoplasms but also can be seen in other cutaneous neoplasms, such as extramammary Paget disease and squamous cell carcinoma (SCC) in situ. Methods: Two-hundred cases of non-melanocytic cutaneous neoplasm, including basal cell carcinomas (BCCs) (n = 41), SCCs (n = 35), Merkel cell carcinomas (MCCs) (n = 25), and adnexal neoplasms (n = 99), were tested for TRPS1 expression using a monoclonal anti- TRPS1 rabbit anti-human antibody. Results: TRPS1 expression was present in almost all cases of SCC (94%), with a median H-score of 200, while it was either absent or only focally present in most BCCs (90%), with a median H-score of 5. The difference between BCCs and SCCs in H-score was significant (p < .001). All MCCs (100%) lacked TRPS1 expression. TRPS1 expression was frequently seen in most adnexal neoplasms, benign and malignant, in variable intensity and proportion but was consistently absent in apocrine carcinomas. All endocrine mucin-producing sweat gland carcinomas (EMPSGCs) (100%, 6/6) showed diffuse and strong TRPS1 immunoreactivity, with a median H-score of 300, which was significantly different (p < .001) than that of BCCs. Conclusions Our study shows that TRPS1 may be an effective discriminatory marker for BCCs and SCCs. It also has a role in distinguishing BCCs from EMPSGCs.