The IL-10 receptor is composed of at least two subunits, both members of the interferon receptor (IFNR) family (Liu et al. 1994). Interferon-10 receptor alpha chain (IL10RA) is the ligand-binding subunit, binding IL-10 with high affinity (Kd 35-200 pM) (Tan et al. 1993). IL10RA is constitutively associated with JAK1 (Moore et al. 2001, Usacheva et al. 2002). This association is dependent on a membrane-proximal part of the receptor (amino acids 269-274) which contain a region designated the box 2B motif, characterized by a core of four hydrophobic residues flanked by a serine and charged residues (Usacheva et al. 2002).