Immunoglobulin G (IgG) is the most abundant Ig isotype in serum, making up approximately 80% of all serum Immunoglobulins. In mice, the IgG subclasses are defined IgG1, IgG2a/c, IgG2b, and IgG3. The IgG2a allele is expressed in e.g., Balb/c mice.