Density-dependent activation of endogenous Notch signaling as indicated by quantitative RT-PCR of Hes1 mRNA. CD34⁺CD38^– cord blood precursors were incubated with increasing densities of Delta1^ext-IgG and expression of the Notch target gene, Hes1, was measured after 6 hours by quantitative RT-PCR (SYBR-green). Results indicate the fold increase in Hes1 levels normalized to control human IgG at increasing densities of Delta1^ext-IgG. Hes1 levels were corrected for mRNA levels based on housekeeping gene expression (R = 0.92; 95% CI, 0.86-0.97; P < .001). Results are representative of 3 experiments ± SEM.