Reading a signalling circuit

In engineered cells, ligand L activates receptor R, which activates kinase K, which activates transcription factor T. Active T increases reporter expression. Without L, baseline reporter is 1 unit; with L, wild type is 16. R-null cells with L give 1; constitutively active K in R-null cells gives 16; constitutively active T in K-null cells gives 16. A compound Z lowers wild-type + L to 4, active-K/R-null to 4, and active-T/K-null to 16. Z leaves the no-ligand reporter baseline at 1 unit. Reporter protein stability and cell number are unchanged. Constitutively active means activity no longer requires the upstream signal.