The diagnostic dilemma is compounded by the documented phenomenon of aberrant HMB45 expression in otherwise conventional leiomyomas.<h4>Patient concerns</h4>A 47-year-old woman presented with an incidentally discovered right retroperitoneal mass.<h4>Diagnoses</h4>Following complete surgical resection, pathological examination revealed a tumor composed of uniform epithelioid cells with minimal atypia, exceptionally low mitotic activity (1/entire specimen), and low Ki-67 index (2%). The gene discussed is MKI67; the disease is leiomyoma.