Analysis of the I-J₁ mutation in the proband and her immediate relatives. An apparent 11-bp deletion is seen in the PCR product of the proband, ie, a 69-bp fragment instead of the normal 80-bp fragment. The mother and the brother are heterozygous and have both 69- and 80-bp fragments. The father, who is an obligate carrier, shows only the 80-bp fragment, which is consistent with a normal genotype.