Benign Sweat-Gland Tubular Adenoma Harboring an MYBL1::NFIB Fusion Gene

Sweat gland (SG) neoplasms encompass a wide variety of benign or malignant adnexal skin tumors differentiating into the various segments of SG. Their diagnosis and classification are often difficult, and have relied mainly on histologic and immunohistochemical criteria. Several genomic alterations have been more recently discovered in SG tumors, and have been used as additional diagnostic criteria. Among them, fusions of the MYBL1 or the MYB gene (namely MYB::NFIB or MYBL1::NFIB) have been detected in a significant proportion of cutaneous and extracutaneous (breast and salivary gland) adenoid cystic carcinomas (AdCC), and have been regarded specific of this tumor type. We report here an exceedingly rare case of a benign neoplasm diagnosed as benign tubular SG adenoma that was unexpectedly found to harbor a MYBL1::NFIB fusion gene. This case expands the spectrum of SG tumors driven by fusions of MYBL1, and challenges the specificity of MYBL1 fusions for cutaneous AdCC.