Mammalian cell lines were constructed that allow for independent modulation of the levels of cis- and trans-Delta and quantitative monitoring of the transcriptional response of a Noth reporter gene (Citrine fluorescent protein cDNA fused to a Notch responsive promoter). Cell lines stably expressed recombinant human NOTCH1 and contained a DNA encoding a doxycycline-inducible recombinant rat Dll1 (cis-Dll1). Cell lines were grown on plates containing different concentrations of adsorbed recombinant Dll1 (trans-Dll1). In the absence of cis-Dll1 (no doxycycline added), NOTCH1 activation is proportional to the concentration of trans-Delta. Induction of cis-Dll1 expression blocks NOTCH1 activation, and the response to cis-Dll1 is sharp and switch-like.