F actor X (FX) is a vitamin K-dependent plasma protein required for the intrinsic and extrinsic pathways of blood coagulation. FXSanto Domingo is a hereditary FX deficiency which is characterized clinically by a severe bleeding diathesis. The proposita has a FX activity of less than 1% and a FX antigen of less than 5%. We have determined the molecular basis of the defect in the FXSanto Domingo gene by amplification of all eight exons with polymerase chain reaction and subsequent sequence analysis. The patient is homozygous for a G----A transition in exon I at codon -20 (numbering the alanine at the NH2 terminus of the mature protein as +1), resulting in the substitution of arginine for glycine in the carboxy-terminal part of the signal peptide. This amino acid change occurs near the presumed cleavage site of the signal peptidase. We hypothesized that the mutation might prevent cleavage by the signal peptidase which in turn would impair proper secretion of the FX protein. To test this hypothesis, we compared the expression of wild type and mutant FX cDNA in a human kidney cell line. Wild type and mutant constructs in the expression vector pCMV4 were introduced into the human embryonic kidney cell line 293 by calcium phosphate transfection. FX antigen levels in the supernatant of the cells harboring the wild type construct were 2.4 micrograms/10(7) cells per 24 h, whereas antigen levels in media from cells containing the FXSanto Domingo construct were undetectable. No FX antigen was detected in the cell lysates of cells transfected with the mutant construct. To insure that the difference in protein levels was not due to a difference in steady state levels of mRNA, Northern analysis was performed on RNA from the cell lysates of both constructs. The results showed a transcript of the same size, present in roughly equal amounts, in both cases. Thus, the defect in the signal sequence of FXSanto Domingo exerts its effect posttranscriptionally. FXSanto Domingo is the first described example of a bleeding diathesis due to a mutation in the signal sequence.