Protein tyrosine phosphatase 1B (PTP1B) is a crucial regulator of various kinase-driven signaling pathways, including the JAK/STAT pathway1. Recent studies indicate that inhibiting PTP1B may enhance T cell anti-tumor immunity, making it a promising target for cancer immunotherapy2,3. To further understand PTP1B's role in these signaling pathways and support drug discovery efforts, structural insights into its interactions with target proteins are essential. Previous research in our lab has solved the crystal structures of complexes between PTP1B and phospho-peptides derived from the kinase activation loop of all four JAKs4. However, the interactions between PTP1B and intact JAK kinase domains remain to be elucidated. In this work, we expressed and purified JAK1 and JAK2 kinase domains from insect cells and analyzed their phosphorylation statuses by mass spectrometry. We used a catalytically inactive PTP1B substrate-trapping mutant to isolate the JAK-PTP1B complexes and characterized them by size exclusion chromatography and mass photometry. We are currently exploring X-ray crystallography and cryo-electron microscopy approaches to determine the structures of these complexes.