Interleukin-9 receptor (IL9R) binds Tyrosine-protein kinase JAK1 (JAK1); this association is believed to be constitutive (Demoulin et al. 1996, Zhu et al 1997). The association is ascribed to a 98-residue juxta-membrane region within the BOX1 motif of the receptor that contains a Pro-X-Pro sequence preceded by a cluster of hydrophobic residues, a consensus sequence shared by many other cytokine receptors e.g. IL4R, IL7R, IL3R (Murakami et al. 1991, Zhu et al 1997).