This is the N-terminal Ig-like domain found in IL-13Ralpha1 type two cytokine complex. The IL-13Ralpha1 contains an extra N-terminal Ig-like domain not found in other receptors of the the common gamma-chain subfamily. The extra N-terminal IL-13Ralpha1 Ig-like domain contacts the dorsal surfaces of both IL-4 and IL-13. Mutational studies show that the deletion of this domain affects the binding of IL-13 to IL-13Ralpha1 [1].

1: Molecular and structural basis of cytokine receptor pleiotropy in the interleukin-4/13 system. LaPorte SL, Juo ZS, Vaclavikova J, Colf LA, Qi X, Heller NM, Keegan AD, Garcia KC; Cell. 2008;132:259-272. PMID:18243101