In contrast to the relatively ubiquitous expression of JAK1, JAK2 and Tyk2, JAK3 is predominantly expressed in hematopoietic lineage such as NK cells, T cells and B cells[3] and intestinal epithelial cells.[5][6][7] JAK3 functions in signal transduction and interacts with members of the STAT (signal transduction and activators of transcription) family.