Elite plant varieties are complex mosaics of carefully combined loci (haplotypes). It is often desirable to introduce new traits as simple add-ons, without haplotype disintegration by cross breeding, and with short time-to-market. We present targeted mutation breeding (TMB), a new mutation-selection technology that delivers plants with substantial novel sequence variation in specific genes in ~4 months, starting from any desired (elite) seed material. This new variation is then used to select improved trait performance. We illustrate our technology with examples of improved traits in tomato and oilseed rape. Using large allelic series in a hormone response gene, we demonstrate how subtle new polypeptide modifications can improve the growth properties of these crop plants. Our technology has been specifically designed to support practical breeding. TMB has wide potential to improve specific traits in crop plants in a very flexible, quick and economical way. The TMB technology is covered by patent applications owned by Keygene N.V.