A genetic marker associated with yield traits in an elite family of dairy cable has been found. The marker has proven to be useful in distinguishing between sons in spite of the nearly identical pedigree merit. Characterization of the Avall polymorphism detected with cDNA probes for prolactin will determine whether or not the structural variation is in the prolactin 8ene.Applications:Once the structural variation providing a genetic marker has been identified, polymerase chain reaction techniques could be developed to amplify the DNA fragment containing the Avail polymorphism. Genetic marker technology has commercial application in identifying those animals which do not carry favorable genes for traits of economic importance in the dairy industry. Pre-screening test population with marker assisted selection techniques would reduce the number of animals needed to produce an equivalent number of proven bulls. Thus, marker selection would accelerate the genetic progress observed in both male and female populations.