We evaluated four candidate SNPs (GC-NPFFR2 rs137147462, GC-NPFFR2 rs109452259, BRCA1 rs134817801, and DGAT1 p.K232A) previously reported in relation to mastitis or milk production traits, using 10,729 test-day phenotypic records collected over 10 years from 269 Japanese Holstein cows (Bos taurus) enrolled in the national Dairy Herd Improvement (DHI) program. The gene discussed is DGAT1; the disease is mastitis.