Interleukin-21 receptor (IL21R, NILR) can bind Janus kinase 1 (JAK1) (Ozaki et al. 2000) but little or no signaling occurs (Asao et al. 2001) unless IL21R is combined with IL2RG, which is required for a fully signaling capable IL21 receptor complex (Ozaki et al. 2000, Asao et al. 2001, Habib et al. 2002). The heteromeric IL21 receptor complex can activate JAK1 and JAK3.
This is a black box event because the pre-association of IL21R with JAK1 and of IL2RG with JAK3 is inferred from the mechanism of IL2 signaling.