In an immunohistochemical study of 120 treatment-naïve gastric cancer samples, the expression loss of central HR-related proteins, BRCA1, ATM, ATR, mediator of DNA damage checkpoint 1 (MDC1), and meiotic recombination 11 homolog A (MRE11), was investigated in relation to clinicopathologic parameters. The gene discussed is MDC1; the disease is gastric cancer.