In the human population, HTT protein levels can vary at least 2-fold or more between different non-HD individuals (Fodale et al., 2022) and although cases of heterozygous individuals with only one functional copy of the HTT gene are rare, they can be found displaying no clinical HD presentation (Ambrose et al., 1994; Jung et al., 2021). The gene discussed is HTT; the disease is Huntington disease.