We have previously shown how recessive human knockouts can present with surprisingly different phenotypes compared with those with monoallelic pathogenic variants, and this study adds several examples of this phenomenon, e.g., MYH11-related megacystis.14 However, we also note that in several other cases, marked phenotypic heterogeneity was noted even when compared with previously reported cases with biallelic pathogenic variants. The gene discussed is MYH11; the disease is Megacystis.