Among the patients with mammary Paget disease (MPD), PD-L1 expression in lymphocytes was lower in patients with HER2/neu-positive disease (median H-score, 2.0; range, 1.0–6.0) than in patients with HER2/neu-negative disease (median H-score, 40.0; range, 6.0–60.0; p = 0.07). This evidence concerns the gene ERBB2 and mammary Paget disease.