Eight of 13 (62%) cases of sinonasal MM displayed heterogeneous A3B nuclear immunostaining (H‐score range = 1–110; median = 24; Figure 2B) that was mostly diffuse (six of eight, Figure 2A; cases 16M and 24M) and less frequently selective or rare (Figure 2A; case 22M). Here, APOBEC3B is linked to Miyoshi myopathy.