The evidence for the association of 11 genes (ABCG2, BoLA-DRB3, CSN1S1, CSN3, DGAT1, GHR, LGB, LEP, LTF, PRL and STAT5A) with mammary gland phenotype and three genes (IL8RA, TLR4 and BoLA-DRB3) with mastitis resistance or susceptibility has been reported more than once in different studies. The gene discussed is PRL; the disease is mastitis.