PER2 and obesity due to melanocortin 4 receptor deficiency: Adipocyte O-GlcNAcylation may promote obesity through its well-established target clock proteins, such as BMAL1, CLOCK and PER2 (Durgan et al., 2011; Kim et al., 2012; Kaasik et al., 2013; Li et al., 2013, 2019; Liu et al., 2021).