Two processes, mutation and paramutation, are postulated to account for the observed heritable changes in the immediate R region of chromosome 10 in maize. Mutation is sporadic, undirected, and yields stable alleles having distinctive effects on aleurone and plant pigmentation. Paramutation occurs invariably in certain heterozygotes, is directed, and gives unstable alleles conditioning intergrading phenotypes. The paramutant phenotype is R-locus dependent, but paramutagenicity and pigment production are separable functions. Paramutation occurs in somatic cells; and paramutable R alleles are widespread in Zea mays. An R factor in a heterozygote with a strongly paramutagenic allele may itself become weakly paramutagenic. Reciprocal translocations involving chromosome breaks at remote points, proximal or distal to R, and in coupling with the latter, increase pigment-producing action of R, and render R comparatively insensitive to paramutation in Rst heterozygotes. These effects are not immediately detectable...