Based on these findings, we therefore suggest that the most useful immunohistochemical approach to differentiate IBM from PM in a muscle biopsy is to use a panel of LC3 and TDP-43 antibodies: a cutoff of <14%FS LC3 helps rule out IBM, while >7%FS TDP-43 strongly supports a diagnosis of IBM. The gene discussed is MAP1LC3A; the disease is inclusion body myositis.