FKBP4 and obesity due to melanocortin 4 receptor deficiency: FKBP51 has emerged asa promising drug target for chronic pain,36 obesity,37,38 and depression.39,40 The major challenge in FKBP51 drug development is selectivity towardits closest homologue FKBP52, as the binding pockets of both proteinsare very similar, but the biological functions are opposite.