In human blood monocytes, where the type I interleukin-4 (IL4) receptor (IL4R1) is the predominant IL4 receptor type, IL4 leads to tyrosine phosphorylation of Janus kinase 1 (JAK1), rather than constitutively bound JAK2, JAK3 or Non-receptor associated tyrosine kinase 2 (TYK2). JAK1 was found to be essential for IL4-mediated expression of 15-lipoxygenase while JAK2 and TYK2 antisense inhibition had no effect (Bhattacharjee et al. 2013). As JAK1 binds IL4RA in response to IL13 binding (Roy et al. 2002), it is believed that IL4 similarly triggers binding of JAK1 to IL4 receptor complexes. The molecular trigger for JAK1 binding is not clear.