The interaction of the mutant genes wellhaarig (we) and waved alopecia (wal) in mice was earlier demonstrated in our laboratory. The we gene significantly accelerates the appearance of alopecia in double we/wewal/wal homozygotes as compared to that in single +/+ wal/wal homozygotes. It has been found in this work that the mutant gene angora-Y (Fgf5(go-Y)) weakens the effect of interaction of the we and wal genes. The first signs of alopecia appear in mice of the we/wewal/wal genotype at the age of 14 days, in triple FgfS(go-Y)/Fgf5(go-Y) we/wewal/wal homozygotes alopecia is observed seven days later, i. e., in 21-day-old animals. The progression of alopecia in triple homozygotes is expressed to a lesser degree than in double +/+ we/wewal/wal homozygotes. A single dose of the Fgf5(go-Y) gene also decreases the effect of interaction of the we and wal genes, but less than a double dose of this gene. The first signs of alopecia in mice of the +/Fgf5(go-Y) we/wewal/wal genotype appear only three days later than in double +/+ we/wewal/wal homozygotes. The data obtained demonstrate that the Fgf5(go) gene is a powerful modifier of mutant genes determining the process of alopecia.