Somatic recombination in the IGH gene of B cells is used to make cell-specific immunoglobulin heavy chains. The human IGH gene has multiple but slightly different repeats for each of four types of gene segments: V (first part of variable region), D (diversity region), J (joining region), and C (constant region; each of the C gene segments has a coding sequence split by introns). An immunoglobulin heavy chain is made by bringing together coding sequences from one each of these four types of segments (shown here as filled boxes). Two sequential somatic recombinations produce first D-J joining, then a mature, functional VDJ coding sequence unit, which is effectively a large novel exon. Another type of somatic recombination (known as class switching can change the position of C gene segments so that other C gene segments can be used instead of Cμ to give alternative classes of immunoglobulin.