HPCA and Dystonia: In humans, autosomal‐recessive mutations in HPCA (OMIM 142622) cause childhood‐onset dystonia and the encoded protein, hippocalcin, is robustly expressed in Purkinje cells and serves as a Ca2+ sensor (Charlesworth et al., 2015; Tzingounis, Kobayashi, Takamatsu, & Nicoll, 2007).