Combined deficiency in vitamin K-dependent (VKD) coagulation factors is an autosomal recessive bleeding disorder associated with defects in either the VKD carboxylase which converts Glus to Glas in VKD proteins to render them active or the vitamin K epoxide reductase (VKORC1) which supplies the reduced vitamin K cofactor required for carboxylation. Such defects are rare, and we now report the fourth case of deficiency caused by mutations in the carboxylase gene. The mutations were identified in a two year old Tunisian girl who exhibited impaired function in several VKD procoagulant and anticoagulant factors that was not restored by vitamin K administration. Sequence analysis of the propositus did not identify any mutations in the VKORC1 gene but, remarkably, revealed three heterozygous mutations in the carboxylase gene, D31N, W157R and T591K within exons 2, 4 and 13, respectively. None of these mutations have previously been reported. Family analysis showed that D31N and T591K were coallelic and transmitted by the mother while W157R was transmitted by the father. The mutations were not found in the genomes of 200 normal subjects, ruling out frequent polymorphisms. Mutational analysis indicated wild type activity for the D31N carboxylase. In contrast, the W157R and T591K enzymes had activities that were, respectively, 7% and 0% that of wild type carboxylase, and their compound heterozygosity can therefore account for defective carboxylation. Residues 157 and 591 are both highly evolutionarily conserved, and residue 157 lies within a region previously suggested to be important for carboxylase binding to VKD Glus or propeptide. However, the hydrophobic nature of this region and inability of vitamin K administration to restore VKD protein function alternatively suggests that residue 157 may be important for vitamin K binding.