TP53 and microcephaly: Furthermore, mice and humans with germline increases in p53 activity can also exhibit microcephaly or cerebellar hypoplasia (Simeonova et al., 2013; Toki et al., 2018), and cerebellar hypoplasia can be observed in a subset of patients with bone marrow failure syndromes, including patients with Hoyeraal–Hreidarsson syndrome (Hoyeraal et al., 1970; Hreidarsson et al., 1988) or 27% of patients with Fanconi anemia (Fiesco-Roa et al., 2019).