Issues

Cover picture: Immunoglobulin (Ig) switch recombination is restricted to normal B cells and a subset of B cell lines. It was hypothesized that a single recombinase was responsible for isotype class switching and that isotype specificity was acquired through germline transcript expression. A new switch plasmid assay indicates that an IgA-switching B cell line supports recombination only on switch plasmids and not on 3 plasmids, whereas both of these plasmids undergo switching in lipopolysaccharide-activated normal B cells. T cell lines showed no switching on either switch substrate. These new studies suggest the presence of distinct factors that distinguish between and 3 recombination events. See related article in this issue by Shanmugam et al., 1365.