sentences:  Finally, by overlapping cDNA cloned fragments, they were able to determine that the C4 loci, an estimated 16 kilobase (kb) long, are spaced by 10 kb and aligned 30 kb from the factor B locus.In the same year, studies relatedly identified a 98 kb region of the chromosome the four class III genes (that express C4A, C4B, C2, and factor B) are closely linked, which does not allow for cross-overs to occur.Using protein variants visualized by electrophoresis, the four structural genes were located between HLA-B and HLA-D. More specifically, they verified the proposed molecular map in which the gene order went from factor B, C4B, C4A, and C2 with C2 nearest to HLA-B.