MAPK4 and obesity due to melanocortin 4 receptor deficiency: mTORC1 signalling in immune cells is reduced in obesity; however, a recent study of The Cancer Genome Atlas (TCGA) PanCancer atlas identified a novel activator of mTORC1—MAPK4 (ERK4), an atypical MAPK that lacks the canonical Thr-X-Tyr (TxY) activation motif for phosphorylation by the dual Ser/Thr and Tyr MAPK kinase (MAPKK) [69,70], which was associated with poor prognosis in a panel of lung, bladder and central nervous system (CNS) cancers, none of which are obesity-related [71].