PGP and obesity due to melanocortin 4 receptor deficiency: Pgp could thus be linked to lipid remodeling in the liver; mice in which both mdr1 genes (as opposed to humans who only have one, mice carry two such highly homologous genes) had been knocked out (mdr1ab-/-) displayed a dramatic retention of TGs in their livers and developed liver steatosis and obesity following a 25- or 35-week regimen of either standard or high-fat diet, despite having a genetic background otherwise resistant to diet-induced obesity [18].