A new mutable gene, mutable miniature wing (mu), is described. This mutable gene mutates spontaneously at an inordinate rate both germinally and somatically. Two classes of reversions of mu have been found in approximately equal frequency: those to an allele with an intermediate phenotype (mi) and those to a subliminal allele (ms) equivalent to wild type in phenotype. Reversions appear to be mutationally stable. The chronology of genetic events leading to the discovery of mu implicates, but does not prove, the insertion of a "foreign" DNA segment as the basis of mutability.