Mutations responsible for HHS have been identified in RTEL1 (regulator of telomere elongation helicase 1), which encodes a helicase that is essential for telomere maintenance, and ACD (adrenocortical dysplasia homolog), which encodes the shelterin TPP1 (Box 1) (Kocak et al., 2014; Walne et al., 2013). The gene discussed is ACD; the disease is hypotrichosis 1.