Fig. 5. Alternative models for JAK/STAT receptor complex-ligand interaction. In A the monomeric receptors exist in the inactive state. Ligand binding induces receptor dimerisation and activation. If this model was correct, ectopic ligand expression should result in ectopic blue staining because of ectopic dimerisation, whereas deletion of the ligand should abolish the blue staining. In the alternative model (B), the inactive receptors are dimerised. Ligand induction activates the receptors through conformational modification. If this model is correct, blue staining would be independent of the expression patterns of the ligand. Experiments in Fig. 3 support the second model. Green ovals, cytokine binding module; red ovals fibronectin type III repeats of DOME. The intracellular receptor domain (orange) binds to HOP (JAK). Blue boxes represent and ß-gal mutants. Complementation of the mutant proteins occurs when receptor dimerisation brings them into close proximity.