Adams et al. (2012) revealed the causal mutation of HH1 as "a nonsense mutation in APAF1 . . . , which is predicted to truncate approximately one-third of the encoded APAF1 protein". Because functional APAF1 peptide is required for embryo development, homozygosity for this allele results in natural spontaneous abortion, and, consequently, perceived reduced fertility in carrier bulls that happen to be mated to carrier cows. Adams et al. (2016) published the causal mutation as being "a nonsense mutation in APAF1 (apoptotic protease activating factor 1; APAF1 p.Q579X) within HH1 using whole-genome resequencing of Chief and 3 of his sons. This mutation is predicted to truncate 670 AA (53.7%) of the encoded APAF1 protein that contains a WD40 domain critical to protein–protein interactions."
