The strongest SNP-MHT interaction (unadjusted Pint <0.0004) was observed with rs9358531 6.5kb 5′ of PRL. Furthermore, three SNPs in PLCG2 that had previously been shown to modify the association of MHT use with breast cancer risk were found to modify also the association of MHT use with mammographic density (unadjusted Pint <0.002), but solely among cases (unadjusted Pint SNP×MHT×case-status <0.02). The gene discussed is PLCG2; the disease is breast cancer.