An 18-year-old German woman presented with progressive cerebellar ataxia since 
early childhood, delayed cognitive development, and hypogonadotropic 
hypogonadism. MRI demonstrated diffuse cerebral hypomyelination, cerebellar 
atrophy, and thin corpus callosum; X-ray revealed persistent milk teeth and 
hypoplastic crowns and roots (figure), indicative of 4H syndrome 
(hypomyelination, hypodontia, hypogonadotropic hypogonadism). POLR3B 
sequencing(1) revealed 2 compound heterozygous mutations (C527R [C.1579T>C] and 
the common ancestral V523E [C.1568T>A](2)).