Following antigen encounter, two distinct processes modify immunoglobulin genes. The variable region is diversified by somatic hypermutation while the constant region may be changed by class–switch recombination. Although both genetic events can occur concurrently within germinal centre B cells, there are examples of each occurring independently of the other. Here we compare the contributions of class–switch recombination and somatic hypermutation to the diversification of the serum immunoglobulin repertoire and review evidence that suggests that, despite clear differences, the two processes may share some aspects of their mechanism in common.