Differentiated cells can be experimentally reprogrammed back to pluripotency by nuclear transfer, cell fusion or induced pluripotent stem cell technology. Nuclear transfer and cell fusion can lead to efficient reprogramming of gene expression. The egg and oocyte reprogramming process includes the exchange of somatic proteins for oocyte proteins, the post-translational modification of histones and the demethylation of DNA. These events occur in an ordered manner and on a defined timescale, indicating that reprogramming by nuclear transfer and by cell fusion rely on deterministic processes.