Gene shuffling under the control of endonucleases rearranges the gene coding for the light and heavy chains of an antibody. Gene segments for variable sequences are randomly selected from sets of possible segments and spliced to form the final gene to be transcribed and translated. Furthermore, other rearrangements in gene segments lead to antibody class switching; i.e. the same B cell creates first IgM and then IgG antibodies (Plasma cells have much rough ER.). Variations can also occur at the level of RNA processing, i.e. the insertion of a hydrophobic portion making an IgM membrane bound.