T h e M Z locus (the MZ gene) in Saccharomyces controls the induction of the adaptive enxyme, melez~tase, which cleaves turanose, maltose, sucrose, alpha-methyl-glucoside, and melezitose (Lindegren and Lindegren, 1953; Palleroni and Lindegren, 1953; Lindegren, Pittman, and Ranganathan, 1957; Lindegren and Pittman, 1958). T h e orderly and limited set of recessive alleles at the MZ locus are designated: TRIISGZ, TMSgZ, TMSgz, TMsgz, Tmsgz, tmsgz. (The capital letters indicate the inductors to which the allele responds; the small letters indicate inability to respond to the corresponding inducer.) Since only these six recessive alleles were discovered, it was assumed that each defect (indicated by a small letter) was due to some sequential change at the locus; otherwise, 31 recessive types should appear. (The only exception to apparent "linearity" is TAlISgZ; the worlting hypothesis assumes that the sequence does not relate to a linear order, since some recon~binations are nonreciprocal.) T h e different recessive alleles can be restored to a functional state in an "all-or-none" fashion as the result of a single mutational event (Lindegren and Pittman, 1958). T h e present communication deals with the genetic analysis of an unusual phenotype, a turanose-, sucrose-, alpha-methyl-glucoside-, melezitose-positive but maltose-negative segregant. T h e genotypic designation which one might assign this mutant is TrnSGZ, but this genotype contradicts the concept of the locus as an orderly sequence and further analysis of the mutant was required. These analyses revealed that the culture xvas the double nlutant TAdSgZ, MG carrying in addition to an allele of MZ, the MG gene which controls the induction of a specific alpha-methyl glucosidase incapable of splitting other alphaglucosides (Hestrin and Lindegren, 1952). But the interactions between the two different alleles in this culture were such that the presence of the MG gene prevented the TMSgZ allele from responding to maltose effectively as an inducer of enzyme. Other TMSgZ alleles which have been studied responded to nlaltose with the production of melezitase in the presence of the A4G gene. This particular AlIZ allele was descended fro111 a totally recessive (mz) culture w11icl1 had niutated to n~elezitose-positive (MZ) by X-ray treatment and exposure to ~nelezitose (Fig. 1) (Lindegren, Pittman, and Ranganathan, 1917). Materials and Methods Czdtz~r-es: T h e unusual segregant was first observed in a family of five tetrads (T80-T119) (Fig. 1 and Table I ) . Dr. D. D. Pitt~nan had collected 58 asci obtained by outcrossing the i\lIZ lnutant with an nlz culture. T h e phenotypes of the dominant cultures are indicated by three capital lettcr designations for the specific sugars fermented, and three small letter designations for each inability. T h e inferred genotypes are indicated after each phenotype. T w o cultures from Pittman's collection, 13.3 (MEZ) and 38.1 (nlez), were mated bv Mr. Tadashi Yoshida to produce Family 1, and the segregants of five tetrads from this family are characterized in Table I. These