Structure and function of the wild-type and I22-inverted F8 loci: DNA, mRNA, and protein. (A) The wild-type F8 locus is comprised of 2 separately regulated transcription units that direct synthesis of 2 F8-exonic-sequence–containing polyadenylated transcript variants, 1 and 2, which are designated here as F8_FL and F8_B, respectively. F8_FL contains all 9030 bases found in F8 exons 1 to 26 and encodes the full-length FVIII protein, FVIII_FL, whose mature circulating form, containing 2332 amino acid residues, is essential for normal blood coagulation. F8_B contains 2598 bases —including 169 bases from its first exon, which is not found in F8_FL, and 2429 bases from its last 4 exons (exons 2-5), which are identical to exons 23 to 26 of F8_FL—and encodes wild-type FVIII_B, a recently identified intracellular protein comprised of 216 amino acids whose function, if any, remains unknown. Homologous recombination between int22h1 and int22h3 (depicted as green and dark blue rectangles, respectively) incompletely inverts F8_FL. (B) The I22-inverted F8 locus is also comprised of 2 separately regulated transcription units that direct synthesis of 2 F8-exonic-sequence–containing polyadenylated transcript variants, 1 and 2, which are designated here as F8_I22I and F8_B, respectively. Because homologous recombination restores the genomic DNA sequence within which it occurs, it has long been suspected,²  but only recently proven experimentally,⁵  that the F8_B gene and transcript, and hence the FVIII_B protein, are identical to that in healthy persons without HA. F8_I22I contains the 6756 bases found in F8 exons 1 to 22 and, together with the 48 additional DCS bases in exon 23_C, encodes the FVIII_I22I protein, which contains 2159 translated amino acid residues and is trapped intracellularly.