Fig4N. [doi:10.1186/1471-213X-7-92] [PMID:17683524] Copyright: This image is from Hou J; Charters AM; Lee SC; Zhao Y; Wu MK; Jones SJ; Marra MA; Hoodless PA. BMC Dev Biol 2007;7:92, an open-access article distributed under the terms of the Creative Commons Attribution License