PCSK1 and obesity due to melanocortin 4 receptor deficiency: Currently, four genes are reported to be related to enteroendocrine cell dysfunction, including NEUROGS (causing congenital malabsorptive diarrhea 4), PCSK1 (causing obesity with impaired prohormone processing/proprotein convertase 1/3 deficiency, or PCSK1 deficiency) (Younis et al., 2020), and genes causing a syndromic disease with diarrhea—ARX (leading to X-linked lissencephaly 2 or hydranencephaly with abnormal genitalia) and RFX (leading to Mitchell-Riley syndrome) (Thiagarajah et al., 2018; Elkadri, 2020).