We found that the level of MGRN1 expression, alone or combined with the differentiation markers PMEL, MLANA, and TYRP1, (i) provided prognostic information on the OS of MM patients with unprecedented accuracy, (ii) was associated with specific transcriptomic landscapes across MM, with differential expression co-occurring with dysregulation of gene sets involved in immunological responses, cell cycle, and DNA damage/repair, and (iii) identified a subset of patients with unexpectedly short OS based on their TNM stage. The gene discussed is MGRN1; the disease is Miyoshi myopathy.