Erb, an allele to Era, and evidence for a third allele, Er.

An antibody against a low-incidence antigen was detected in the serum of a woman whose newborn infant was found to have a positive direct antiglobulin test. The antibody failed to agglutinate 79 examples of red cells having low-incidence antigens and 16 examples of high-incidence antigen-negative red cells. The woman's serum reacted strongly with her husband's red cells in the antiglobulin test and with 5 of 6 Er(a-) cell samples from unrelated donors, suggesting that the antigen has an antithetical relationship to Era. The failure of the serum to react with one Er(a-) cell sample and with cells from the Er(a+) daughter of an Er(a-b+) mother gives evidence for a silent allele, Er. Four Er(b+) bloods were found among 605 random white donors, indicating a gene frequency for Erb of 0.0033.