Somatic crossing over in flies heterozygous for a dominant Minute mutant may result in two cells, homozygous for wild type and Minute, respectively. If both cells took part in development, a twin spot of two genotypes, different from the rest of the body, would result. Two-hundred wild type spots were found, none of which was accompanied by an adjacent twin spot homozygous for Minute. The absence of these spots shows that their cell lethality is not overcome by transport, including diffusion, across a few cells of gene-dependent material from the neighboring cells which carry one or two wild type alleles of Minute.