HCK and arthritic joint disease: Whenever tested, deletion of the signalling proteins from the haematopoietic compartment was sufficient to abolish arthritis development,99,115,120,121 and in the case of Syk, it has even been shown to be due to Syk deletion in the neutrophil compartment.121 Importantly, the Hck–/–Fgr–/–Lyn–/–, Syk–/–, PLCγ2–/–, and Vav1–/–Vav2–/–Vav3–/– mutations all caused complete defects in leucocyte recruitment to the site of inflammation99,115,120,122 and the overall phenotype strongly resembled that of CD18–/– mice lacking all β2-integrins.123,124