Signaling through human CRLF2 involves JAK2. (A) CRLF2 signaling can be activated by either ligand or gain-of-function mutations within CRLF2, IL7R, JAK1, or JAK2. On the left, CRLF2 can heterodimerize with IL7R and respond to TSLP through JAK1 and JAK2. In the middle, CRLF2 can signal through a gain-of-function mutant JAK2, in which case involvement of a dimerization partner is unknown. On the right, CRLF2 F232C can homodimerize through a disulfide bond.¹⁸  C-terminal tyrosines (Y) within CRLF2 and IL7R are indicated. (B) Coimmunoprecipitation of HA-CRLF2 with mouse Jak2 R683 (wild-type), R683G, and R683S in Ba/F3 lysates. Coimmunoprecipitation of wild-type Jak2 and βc is included as a control. (C) Coimmunoprecipitation of FLAG-tagged human JAK2 with CRLF2 or CRLF2 F232C in Ba/F3 lysates. (D) A series of mutation and deletion constructs were generated in CRLF2 to define the role of specific residues (F232, W286, Y368) or domains (Box 1, cytoplasmic, extracellular). Modified amino acids are underlined. Numbers above or below constructs indicate codons. (E) Coimmunoprecipitation of GST or GST-CRLF2-INT generated in E coli with HA-JAK2 (wild-type, WT; R683G or V617F) generated by in vitro translation.