Cytokine signalling plays a key role in the regulation of haematopoiesis. When haematopoietic cytokines bind to their respective receptors at the cell membrane of target cells, they trigger the activation of downstream cytosolic signalling cascades such as the JAK/STAT, PI3K and MAPK pathways which lead to the transcription of genes promoting cell proliferation and differentiation. Granulocyte colony stimulating factor (GCSF) is a haematopoietic cytokine that stimulates the production of neutrophils and is used to treat cancer patients suffering from chemotherapy-induced neutropenia. The GCSF receptor (GCSFR) is a member of the tall cytokine receptor family characterised by an N-terminal Ig-like domain and three membrane proximal fibronectin type III (FnIII) domains in the ectodomain that are important for signal transduction. The crystal structure of the related tall receptor gp130 shows that the FnIII legs adopt a bent conformation in the unbound state and although the legs approach one another they don’t appear to interact directly. This bend may promote signal transduction by bringing constitutively bound JAKs on the receptor’s cytosolic tail into a position where they can phosphorylate each other and their opposing receptors. Residues responsible for the bend in the gp130 FnIII legs are conserved between members of the tall receptor family including GCSFR. To determine whether the FnIII legs of the GCSFR adopt the same bent conformation as those of gp130 and whether they undergo a conformational change upon ligand binding, I aim to use X-ray crystallography to solve the structure of the GCSFR ectodomain in ligand-bound and unbound states. Here I have expressed Fc-tagged recombinant GCSF and GCSFR ectodomain in mammalian HEK 293 freestyle cells and shown using size exclusion chromatography that they are able to bind each other.