Chemical derivatization of the terminal phosphate group of DNA, as well as the backbone linker phosphates, with sulfur-containing functional groups were developed for covalent attachment of DNA on gold surfaces via chemisorption. Results of IR measurements and quartz crystal microbalance analyses for the DNA-modified gold substrates were described and the immobilization chemistry of the present system was discussed. Next, the techniques of DNA immobilization were applied to develop a biosensor that can discriminate and respond to specific genes (gene sensor). Two examples of gene sensors both of which were coupled with electrochemical signal readouts have been successfully introduced; the first example of gene sensor was taking advantage of a redox-active psoralen compound that binds covalently with DNA and the next was achieved by the use of redox active surface adlayer having a DNA/ferrocene/Au interfacial structure. The basic feasibility of the gene sensor was also discussed.