McC/Sl^a haplotypes were determined by Western blot to resolve typing discrepancies.

Donor no. 011 (Table 8) exhibits weaker expression of the 190-kd form and normal expression of the 220-kd protein (lane 2) when probed with the standard anti-CR1 (J3D3). Probing with the monoclonal anti-McC^a (6B1.H12) shows that only the 220-kd protein carries McC^a (lane 1). Therefore, the 190-kd form carries McC^b, and because of its weak expression, a false-negative result is obtained for serologic typing. Donor no. 008 (Table 8) has a low total RBC-CR1 level (186 copies/RBC) and exhibits weaker expression of the 250-kd form compared with the 220-kd form (lane 4). With the use of MAb 3C6.D11, the 250-kd protein is shown to carry both McC^a and Sl^a (lane 3). The low expression of this allele contributes to the false-negative serologic typing for both McC^a and Sl^a. Molecular weights are noted to the side of each allele, and the genotypes for positions 1590 (McC) and 1601 (Sla) are in parentheses.