TTC21B and obesity due to melanocortin 4 receptor deficiency: Specific to IFT139, perinatal loss of IFT139 can lead to obesity in adulthood (Jacobs et al., 2016, 2020), and global deletion of IFT139 in juvenile or adult mice can lead to cystic kidneys that are reminiscent of autosomal dominant polycystic kidney disease (Tran et al., 2014; Wang et al., 2022).