The methylation and demethylation of the 5-position in cytosine, which stops the expression of the gene in vertebrates, is passed along on one strand of DNA. The daughter cells may spontaneously methylate (at rate ) or demethylate (at rate ). At the step of successive divisions, the probabilities of the cell having one, two, or none of its labile sites methylated are denoted by , , and , which give the proportion of homomethylated, hemi-methylated, and unmethylated sites, respectively, in generation .