N^opa32 is a natural variant found in FM4 and FM6.

Like FM4 except for the presence of the additional In(1)15D-E;20A-B. Reservations similar to those about the balancing ability of FM4 apply in FM6 to the salivary chromosome region from 1B to 15D. In genotypes with a normal recombination rate, FM6 effectively eliminates recombination in FM6/+ heterozygotes but yields viable recombinants when heterozygous for such inversions as In(1)sc⁸. Used for balancing sex-linked recessive lethal and sterile mutations. Does not effectively balance cv or v in stocks that are also heterozygous for In(2LR)SM and In(3LR)TM2. In addition recessive-lethal-bearing FM6 chromosomes have been described (e.g., FBrf0019966).