The development of neutralizing antibodies against plasma-derived or recombinant (r) molecules of wildtype (wt) factor (F)VIII protein is the most serious complication of replacement therapy for hemophilia A (hA) patients. Although the pathogenesis of these antibodies, termed inhibitors, is complex and poorly understood, ethnicity, a recently established risk determinant, is clearly involved since African-American (AA) patients experience this complication ~2-fold more often than Caucasians. In a previous study -- in which the FVIII genes (F8) from 137 unrelated healthy people, representing 7 ethnic groups, were resequenced -- we identified 4 common nonsynonymous single nucleotide polymorphisms (nsSNPs) and thereby demonstrated that FVIII is not a monomorphic protein in non-hemophiliacs. Interestingly, 5 of the 6 distinct wt proteins encoded by the allelic combinations or haplotypes (H) of these nsSNPs, designated H1, H2, ..., H5, are expressed by AA’s compared to only 2 in Caucasians (H1 and H2). Because H3, H4 and H5 are 1) partially defined by AA-restricted minor alleles of R484H and M2238V, 2 nsSNPs that substitute amino acids in the A2 and C2 major B-cell inhibitor epitopes, 2) represent the wt FVIII protein in ~27% of AA’s, and 3) differ from the rFVIII proteins used clinically, we designed the PIR study -- the 1st cross-sectional study of AA hA patients -- to determine if pharmacogenetic factors contribute to their greater inhibitor risk. Because the strongest known risk factor for inhibitors is the F8 mutation type, we are resequencing the known functional regions of F8 in the 1st 50 enrolled PIR subjects, out of the 223 total AA hA patients treated at the six participating Region IV South hemophilia treatment centers, to test the plausible alternative hypothesis that the higher inhibitor incidence is due to a different spectrum of molecular abnormalities than has been found in other ethnic subgroups of patients already examined at the DNA sequence level. We have 1) determined the plasma FVIII-activity and -antigen levels, 2) identified the causative mutation in 18 patients, several of which are novel, and 3) are currently performing RT-PCR analyses for detecting the common inversions in introns 1 and 22. Once completed, we will statistically compare the prevalence of AA F8 mutation subtypes to that in hA patients from other ethnic subgroups (detailed in the HAMSTeRS database), and the proportion of AA FVIII deficiencies that are 1) severe, moderate and mild, and 2) CRM-positive and -negative.