RNU4ATAC and microcephalic osteodysplastic primordial dwarfism type I: We identified rare compound heterozygous variants disrupting highly conserved elements of the small nuclear RNA (snRNA) gene RNU4ATAC (RefSeq NR_023343, OMIM 601428), which is essential for minor intron splicing8, 9, 10, 11 and was reported to cause the recessive disorder microcephalic osteodysplastic primordial dwarfism, type I (MOPD1, OMIM 210710) (refs 10, 11).