A cellular marker for individual somatic cells and their clonal descendents would be a valuable tool for the investigation of cell lineages and clonal organization in developing and in renewing tissues. Such markers have been developed in Drosophila¹, but (apart from mutant melanocytes in retinal pigmented epithelium^2,3) not so far in mammalian tissues. We report here the development of a mutation-induced marker in mice heterozygous at the Dlb-1 locus which determines the expression of binding sites for the lectin Dolichos biflorus agglutinin (DBA) in intestinal epithelium⁴. We show that this marker can be used to study the clonal organization of adult intestinal epithelium, and to mark descendent clones arising during development. The method can in principle be extended to any other suitable markers which can be obtained in a heterozygous state, including markers generated in transgenic animals.