Differential sensitivity of Notch target genes in response to changes in Notch-signaling intensity. Quantitative real-time RT-PCR gene expression analysis of Notch and T cell–related genes in human postnatal CD34⁺ thymocytes, either freshly isolated (day 0) or after 40 hours of culture on OP9-DL1 and OP9-GFP stromal cells in the presence of various concentrations of GSI. Data shown are the mean plus or minus SEM from 2 sets of independent samples and expression levels for all genes are presented in units relative to those from CD34⁺ cells from the same donor cultured on OP9-DL1 cells in the absence of GSI and normalized to the geometric mean of β-actin, YWHAZ, and GAPDH.⁵⁰