Immunoglobulin heavy chain (IgH) class switch recombination (CSR) replaces the initially expressed IgH Cmu exons with a set of downstream IgH constant region (C(H)) exons. Individual sets of C(H) exons are flanked upstream by long (1-10-kb) repetitive switch (S) regions, with CSR involving a deletional recombination event between the donor Smu region and a downstream S region. Targeting CSR to specific S regions might be mediated by S region-specific factors. To test the role of endogenous S region sequences in targeting specific CSR events, we generated mutant B cells in which the endogenous 10-kb Sgamma1 region was replaced with wild-type (WT) or synthetic 2-kb Sgamma3 sequences or a synthetic 2-kb Sgamma1 sequence. We found that both the inserted endogenous and synthetic Sgamma3 sequences functioned similarly to a size-matched synthetic Sgamma1 sequence to mediate substantial CSR to IgG1 in mutant B cells activated under conditions that stimulate IgG1 switching in WT B cells. We conclude that Sgamma3 can function similarly to Sgamma1 in mediating endogenous CSR to IgG1. The approach that we have developed will facilitate assays for IgH isotype-specific functions of other endogenous S regions.