During cellular differentiation unspecialized cells take on characteristic features, such as shape, size, or membrane protein composition, to perform distinct functions in the body. A fertilized egg or zygote is highly proliferative and totipotent; that is, it can differentiate to form all cell types. As the zygote undergoes a series of divisions, the daughter cells lose their totipotency and differentiate to form specialized cells in the embryo, such as muscle cells, neurons, skin epidermis, heart cells, and liver cells. Though all these body cells contain the same genome, they express certain genes while switching off others. This allows them to synthesize the proteins necessary to carry out tissue-specific functions.