Azacytidine is an analog of cytidine in which the carbon at position 5 of the pyrimidine ring has been replaced by a nitrogen. Azacytidine is phosphorylated by cells and incorporated into DNA just like cytidine, but azacytidine cannot be methylated at position 5, the sole site of eukaryotic DNA methylation.

Eukaryotic cells treated with azacytidine exhibit changes in gene expression, probably due to the change in methylation patterns arising from incorporation of azacytidine into their DNA.

Interestingly, after the cells treated with azacytidine are allowed to replicate so as to replace the azacytidine residues with cytidine residues, the newly replicated sequences remain unmethylated, even if they were methylated prior to azacytidine treatment.

For example, azacytidine treatment can cause adult bone marrow cells to reactivate the synthesis of fetal hemoglobin, which is normally turned off during development.