Bimolecular Fluorescence Complementation assay reveals the Hoxa1-PBX1A interaction in culture cells. COS7 cells were transfected with combinations of VN173, VN173-PBX1A, VC155 and VC155-Hoxa1 expression vectors. Upon interaction between PBX1A and Hoxa1, the VN173 and VC155 moieties of the Venus fluorescent protein brought together provide a fluorescent signal (A). Fluorescence complementation does not appear when the VN173 or VC155 fragments are expressed instead of the corresponding fusion proteins (B-C). As a control, expression of Venus fragments is detected by immunocytochemistry (anti-GFP). The BiFC signal shows colocalization with the anti-GFP immunofluorescence (D-F).