PPP1R21 and obesity due to melanocortin 4 receptor deficiency: In this work, it was established for the first time that the BC-risk effect correlated by GWAS with the SHBG-level polymorphism depends on the presence/absence of obesity: The SHBG-lowering allele C rs10454142 PPP1R21 has a risk value for BC in obese women (OR = 1.52–1.95) and is not associated with the disease in women without obesity.