ALDH1A1 and obesity due to melanocortin 4 receptor deficiency: Supporting the hypothesis that ALDH1a1 drives rexinoid activation while ALDH1a2/1a3 drives retinoid activation, ALDH1a1 knockout mice exhibit increased brown fat synthesis and obesity resistance (Kiefer et al., 2012), which phenocopies RXRγ but not RAR knockout (Haugen et al., 2004).