ATP2B1 and Global developmental delay: While the conspicuous neurodevelopmental deficit in the proband with biallelic ATP2B1 variants overlaps with the global developmental delay (mild to moderate) observed in the de novo monoallelic cohort [10], the parents of our proband, heterozygous for pathogenic ATP2B1 variants do not show overt clinical symptoms, including the c.3060+2 T > G variant carrier mother.