The function of a series of LDL receptor GFP fusion proteins with different, flexible, unstructured spacer regions was analysed. An optimised version of the fusion protein was used to analyse the effect of an LDL receptor mutation (W556S) found in FH patients and characterised as transport defective. In cultured liver cells this mutation was found to inhibit the transport of LDL receptor GFP fusion protein to the cell surface, thus leading to impaired internalisation of fluorescent labelled LDL. Co-localisation studies confirmed the retention of the mutant protein in the endoplasmic reticulum. Wild type (WT) and W556S LDL receptor GFP fusion proteins were expressed in mouse liver by means of hydrodynamic delivery of naked DNA. Two days after injection liver samples were analysed for GFP fluorescence. The WT LDL receptor GFP protein was located on the cell surface whereas the W556S LDL receptor GFP protein was retained in intracellular compartments. Thus, the GFP-tagged LDL receptor protein allows both detailed time lapse analysis and evaluations in animals for the physiological modelling of mutations. This method should be generally applicable in functional testing of gene products for aberrant processing.