Total RNA was isolated using the RNeasy Mini kit (Qiagen) and standard cDNA synthesis was performed. Quantitative PCRs (qPCRs) were performed in triplicate. PCR amplification, using SYBR Green, was performed in 96-well microtiter plates in an iCycler thermal cycler (Bio-Rad, Hercules, CA). Sample cDNAs were compared with expression of house-keeping genes Gapdh and actin using the relative quantification ΔΔC_T technique.³⁹ Relative expression levels in the different LSK populations were estimated by first calculating the number of molecules formed at reaching the C_T. By correcting this value for the initial number of cells used, relative expression was determined.