The factor XII genes of two unrelated factor XII-deficient Japanese families were screened, and two novel mutations were identified. A heterozygous mutation (Q421K) was identified in the gene of a cross-reacting material (CRM)-negative patient with reduced FXII activity (entitled Case 1). No mutations were discovered in the other allele. Case 2 was a CRM-negative patient with severe FXII deficiency. In this case, a homozygous mutation (R123P) was discerned. Expression studies in Chinese Hamster Ovary (CHO) cells demonstrated accumulation of mutant Q421 K factor XII in the cell, and insufficient secretion, while the R123P mutant showed lower levels of accumulation than wild-type, and no evidence of secretion in culture supernatant. In the presence of proteasome inhibitor, all types of FXII (wild-type. Q421K, R123P) accumulated in the cells. Protease protection experiments using the microsomal fraction of these cell lines demonstrated that while 20% wild-type FXII (total wild-type:100%) and 10% R123P mutant (total R123P-type: 40%) were resistant to treatment with trypsin, 50% Q421K-type FXII (total Q421K-type:130%) remained resistant to digestion. From these results, we conclude that Q421K is less susceptible to proteasome degradation than wild-type, but is unable to exit the ER efficiently, resulting in insufficient secretion phenotype. In contrast, R123P is susceptible to proteasome degradation and is not secreted.