TPP1 and neuronal ceroid lipofuscinosis: Classic late‐infantile neuronal ceroid lipofuscinosis, CLN2 disease, is the result of tripeptidyl peptidase I (TPP1) deficiency, caused by autosomal recessive inheritance of two pathogenic variants in trans in the TPP1 (MIM# 607998, CLN2) gene (Fietz et al., 2016; Mole, Gardner, Schulz, & Xin, 2018; Sleat et al., 1997).