Microcephalic osteodysplastic primordial dwarfism type I (MOPD I) is a rare 
autosomal recessive developmental disorder characterized by extreme intrauterine 
growth retardation, severe microcephaly, central nervous system abnormalities, 
dysmorphic facial features, skin abnormalities, skeletal changes, limb 
deformations, and early death. Recently, mutations in the RNU4ATAC gene, which 
encodes U4atac, a small nuclear RNA that is a crucial component of the minor 
spliceosome, were found to cause MOPD I. MOPD I is the first disease known to be 
associated with a defect in small nuclear RNAs. We describe here the clinical 
and molecular data for 17 cases of MOPD I, including 15 previously unreported 
cases, all carrying biallelic mutations in the RNU4ATAC gene.