PML and acute promyelocytic leukemia: Class II mutations are those affecting transcriptional regulatory genes, such as the CBF group mutations, which primarily manifest as translocations detectable by fluorescence in situ hybridization (FISH), such as t(8;21), inv(16), and t(15;17), the latter being responsible for the PML-RAR-α gene fusion characteristic of acute promyelocytic leukemia (APL).