Mice

All mice used were on a C57BL/6 background. The Hhex^fl (Hunter et al. 2007), Hhex⁻ (Bogue et al. 2003), Mx-Cre (Kuhn et al. 1995), ROSA26-Cre^ERT2 (Seibler et al. 2003), and Ink4a/Arf^−/− (Serrano et al. 1996) mouse strains have been described. Four-week-old to 6-wk-old CD45.1⁺ C57BL/6 mice (Ly5.1; Walter and Eliza Hall Institute) were used as recipients in chimeric transplant experiments. To induce expression of the Mx1-Cre allele, polyinosinic–polycytidylic acid sodium salt [poly(I:C); Sigma] dissolved in saline was administered to mice (12 mg per kilogram of mouse body weight) intraperitoneally at 7 wk of age at least 3 wk prior to their use in experiments. To induce expression of the ROSA26-Cre^ERT2 allele, mice were administered 70 µL of tamoxifen (4.2 mg [Sigma] in vehicle; 10% ethanol, 90% peanut oil [Sigma]) by oral gavage on two consecutive days, whereas placebo-treated mice received 70 µL of vehicle control. All experiments were approved by the Walter and Eliza Hall Institute Animal Ethics Committee.