The Kn1-2F11 mutation causes protrusions or knots along the lateral veins of the first few leaves of the maize plant. The phenotype is visible when an unlinked gene, presumably Ac, is present in the genome. The mutation is closely linked to a genetically unstable Adh1 mutation that resulted from the insertion of a Ds2 element (Döring et al., 1984; Chen et al., 1986). Using a unique sequence from the Ds2 element as a hybridization probe, a genomic restriction fragment that cosegregated with the knotted phenotype was cloned. It carries the Kn1-2F11 locus by the following criteria. (i) Cosegregation of the fragment is tightly linked to the phenotype. (ii) Somatic and germinal excision produce a fragment which is the expected size of a revertant fragment; progeny containing the revertant size fragment are normal. (iii) The sequences that hybridize to this fragment are significantly altered in the chromosome containing the original knotted mutation, Kn1-O, (iv) The cloned fragment does not hybridize to a chromosome that contains a deletion of Kn1-O.