Nature 478, 255–259 (2011).

In this Letter, two additional references32, 33, which described stem-cell organization in the intestinal crypt, should have been cited after the first sentence of the main text: “Two types of stem cells have been described in the small intestine based on location and cycling dynamics1–4, 32, 33.”. The references and their citations have been added to the PDF and HTML versions (and appear below as refs 1 and 2).