Breeders have traditionally improved plant varieties by selecting on the basis of phenotype. Now restriction fragment length polymorphism (RFLP) linkage maps are being constructed for most major crop plants and these maps provide a more direct method for selecting desirable genes via their linkage to easily detectable RFLP markers. The integration of RFLP techniques into plant breeding promises to: (1) Expedite the movement of desirable genes among varieties, (2) Allow the transfer of novel genes from related wild species, (3) Make possible the analysis of complex polygenic characters as ensembles of single Mendelian factors, and (4) Establish genetic relationships between sexually incompatible crop plants. In the future, high density RFLP maps may also make it possible to clone genes whose products are unknown, such as genes for disease resistance or stress tolerance. © 1989 Nature Publishing Group.