Bone marrow cells were analyzed for retention of biotin labeling as previously described [26 (link)]. Cell surface markers used for analysis were identical to those used for cell cycle analysis with the following exceptions: biotinylated Ryk was replaced with sheep anti-mouse Ryk (AF4649, R&D Systems) with Alexa-Fluor 546-conugated donkey anti-sheep IgG acting as a secondary antibody (Invitrogen, Carlsbad, CA, http://www.invitrogen.com). APC-conjugated streptavidin (eBioscience) was used to detect biotin-labeled cells.