The invention provides a fusion protein comprising, from N-terminus to C-terminus: a) a first portion of a Family B G-protein coupled receptor (GPCR) that comprises transmembrane helix (TM)-1, TM2 and TM3 of the GPCR; b) a stable protein domain; and c) a second portion of the GPCR comprising TM4, TM5, TM6 and TM7 of the GPCR. The invention also provides a method of crystallising a GPCR comprising providing the fusion protein of the invention and crystallising it to obtain crystals.