PER2 and isolated hemihyperplasia: Notably, this patient had IH and a c.1403G>A:p.(Arg468Gln) variant in PER2. PER2 is involved in the mammalian circadian clock system, together with cryptochrome (CRY), circadian locomotor output cycles kaput (CLOCK), and brain and muscle Arnt-like protein 1 (BMAL1) [18–20].