Results from an investigation of the frequency response resulting from ligand binding for a genetically engineered hormone-binding domain of the alpha-estrogen receptor immobilized to a piezoelectric quartz crystal are reported. Two different approaches were taken to attach a genetically altered receptor to the gold electrode on the quartz surface: 1) the mutant receptor containing a single solvent exposed cysteine was directly attached to the crystal via a sulfur to gold covalent bond, forming a self assembled protein monolayer and 2) the N-terminal histidine-tagged end was utilized to attach the receptor via a 3,3-dithiobis [N-(5-amino-5-carboxypentyl)-propionamide-N'-N'-diacetic acid] linker complexed with nickel. Previous studies have shown that these engineered protein constructs bind 17-beta- estradiol and are fully functional. Exposure of the receptor directly attached to the piezoelectric crystal to the known ligand 17-bet- estradiol resulted in a measurable frequency response, consistent with a change in conformation of the receptor with ligand binding. However, no response was observed when the receptor immobilized via the linker was exposed to the same ligand. The presence of the linker between the quartz surface and the protein receptor does not allow the crystal to sense the conformational change in the receptor that occurs with ligand binding. These results illustrate that the immobilization strategy used to bind receptor to the sensor platform is key to eliciting an appropriate response from this biosensor. This study has important implications for the development of QCM and other sensors using protein receptors and gold surfaces.