Carbohydrate deficient-glycoprotein syndrome type I (CDG1 or Jaeken Syndrome) is an autosomal recessive multisystem disease with severe early involvement of the nervous system. Mutations in the phosphomannomutase 2 (PMM2) gene have recently been identified in 16 affected individuals. In the current study, we have described a CDG1 family where gene tracking had been used to perform prenatal diagnosis before the isolation of the CDG1 gene. Haplotype analysis indicated that the unborn child had inherited the maternal 'normal' allele, but a critical recombination event meant that it was impossible to determine if the child had inherited the paternal mutation. Single-strand conformation polymorphism and sequence analysis revealed that the mother was a carrier of a C-->A transversion at position 357 (F119L), and that the father was a carrier of a G-->A transition at position 425 (R141H). The unborn child had inherited the paternal R141H mutation. Because only three mutations have previously been reported in UK families, of which F119L and R141H are two, and given that there is evidence of allelic association in CDG1 families, it is possible that a limited number of ancestral mutations have given rise to most cases of CDG1 in any one population.