Among genetically identical "agouti viable yellow" (Avy) mice, some are brown and lean while others are yellow and obese. These differences are due to epigenetics, a system of molecular tags—such as methyl groups—that are added to DNA during development to determine gene expression in different cell types. A 2003 study showed that simply supplementing the diet of female mice with 'methyl donor' nutrients before and during pregnancy caused more of their Avy offspring to turn out lean and brown.