 Phosphorylation of these dual tyrosines leads to the conformational changes in the JAK protein to facilitate binding of substrate.JH2 is a "pseudokinase domain", a domain structurally similar to a tyrosine kinase and essential for a normal kinase activity, yet lacks enzymatic activity.This domain may be involved in regulating the activity of JH1, and was likely a duplication of the JH1 domain which has undergone mutation post-duplication.