In the latter, two siblings, negative for RNU4ATAC but having compound heterozygous missense variants in RTTN, were clinically diagnosed with MOPD1 as they were presenting severe pre- and post-natal growth delay, severe microcephaly (with cerebellar hypoplasia, dysgenesis/agenesis of corpus callosum, lissencephaly or reduced sulcation, ventricular abnormalities), joint contractures and the typical facial dysmorphism (prominent eyes, sloping forehead, micrognathia). Here, RNU4ATAC is linked to microcephaly.