In the version of this article initially published, there were two errors in the discussion of epigenetic marks on page 854. In sentence 4, paragraph 2 of the section “Genetic and epigenetic changes predictive of malignancy,” dimethylated and trimethylated H3K9 were said incorrectly to be “polycomb” marks. “Polycomb” has been deleted from the sentence, and the following two sentences inserted for clarification: “In ES cells these genes are held in a 'transcription ready' state by two marks, a repressive H3K27me mark and an active mark, H3K4me64. Changes in the balance of repressive versus active marks can alter the activity of these genes, hypothetically keeping cells in a proliferative state.” Further down in the paragraph, DNMT3L was described incorrectly as catalytic. The original text, “... related to activation of the de novo methyltransferase DNMT3L68” has been revised to “...and maintain expression of the de novo methyltransferase–like protein DNMT3L68. Expression of DNMT3L appears to be a common feature in pluripotent cells including ES, EC and embryonic germ cells.” The errors have been corrected in the HTML and PDF versions of the article.