Using absorbance units (AU) to measure anti-cN1A reactivity, an IgG cut-off >0.9 AU had 51 % sensitivity and 94 % specificity, an IgA cut-off >1.2 AU had 49 % sensitivity and 95 % specificity, and an IgM cut-off >1.9 AU had 53 % sensitivity and 96 % specificity for diagnosing IBM in a population of 205 individuals: 50 with IBM, 121 with another neuromuscular disease, and 34 healthy controls. This evidence concerns the gene CD79A and inclusion body myositis.