In Additional file 1: Table S3, a significant effect of obesity on the association between MMP1 haplotypes and MMP1 level was also noted in haplotype interaction analysis after adjusting for age, gender, and smoking, with the association found predominantly in non-obese subjects (P = 2.29x10-6 for haplotype A1GTAG and P = 0.002 for haplotype A2GTAA, and interaction P = 0.003 for A1GTAG). This evidence concerns the gene MMP1 and obesity due to melanocortin 4 receptor deficiency.