Factor V (FV) deficiency is a rare bleeding disorder whose genetic basis has been described in a relatively small number of cases. Among a total of 12 genetic defects reported in severely or moderately severe deficient patients, 3 were missense mutations and in no case was the mechanism underlying the deficiency explored at the molecular level. In this study, a homozygous missense mutation at cDNA position 6394 in exon 23 of the FV gene was identified in a 22-year-old Italian patient. This mutation causes the replacement of arginine 2074 with a cysteine residue (Arg2074Cys) in the C2 domain of the protein. The effect of the Arg2074Cys mutation on FV secretion, stability, and activity was investigated. Site-directed mutagenesis of FV cDNA was used to introduce the identified mutation, and wild-type as well as mutant FV proteins were expressed by transient transfection in COS-1 cells. An enzyme immunoassay detected low FV antigen levels both in the conditioned media of cells expressing the mutant protein and in cell lysates. Metabolic labeling and pulse-chase experiments confirmed that the mutation caused an impaired secretion of FV associated with rapid intracellular degradation. In addition, evaluation of wild-type and mutant coagulant activity demonstrated that the FV molecules carrying the Arg2074Cys mutation have reduced activity. These findings, beside confirming the structural and functional importance of the arginine 2074 residue, demonstrate that its substitution with a cysteine impairs both FV secretion and activity.