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We have been investigating a new approach to realize an clectrochemical detection for DNA chips, although a number of fluorescent detection methods ate widely used for SNP genotyping [l]. The novel concept of a genetic field effect transistor (FET) is proposed in the present study for improving precision, standardization and miniaturization of a DNA chip system. The genetic FET is composed of Si with Si3N4/SiO2 as the gate insulator on which DNA probes are immobilized and subsequently hybridized with target DNA in sample solutions. The potentiometric detection method is based on the direct transduction of surface density change of charged biomolecules into electrical signal by the field effect and is effective for charged species such as DNA molecules. Here, we report the concept of genetic FET and the ability of SNP genotyping by controlling hybridization temperatures, and by the utilization of intercalator or primer extension reaction using the genetic FET.