A human zygote has three immunoglobulin loci, one that specifies the heavy chain and two that specify the light chain. Taking into account differences between maternal and paternal alleles a B cell might be expected to have the potential of making a total of two different heavy chains and four different light chains, and therefore eight different immunoglobulins. Instead, each mature B cell makes just a single type of immunoglobulin. How does that happen?