DHODH and postaxial acrofacial dysostosis: It is therefore reasonable to assume that the threshold for a recognisable Miller syndrome phenotype should be somewhere between 50 % and 0 % total enzymatic activity compared to an individual homozygous for wildtype DHODH. The variant R135C, the common background in both cases discussed here, retains little activity in vitro, but to our knowledge no patient has been described with this variant in homozygous form, but always in compound heterozygosity with another DHODH variant and never in a mild phenotype [15,19,23].