A diagnostic PCR simultaneously using 3 primers for detection of the I-J₂ mutation. F1 and R primers yield a 340-bp fragment only in DNA of a chromosome bearing the mutation as in a homozygous patient (P). In normal DNA (N), no amplification occurs due to the large distance between the primers (<11 kb). F2 and R primers yield a 190-bp fragment only in normal DNA, with no amplification occurring in mutant DNA because F2 is located within the deletion (P). In two hetrozygotes (H), both 190- and 340-bp fragments are demonstrable.