We report a specific, label-free and real-time detection of femtomolar protein concentrations with a novel type of nanowirebased biosensor. The biosensor is based on an electrostatically formed nanowire, which is conceptually different from a conventional silicon nanowire in its confinement potential, charge carrier distribution, surface states, dopant distribution, moveable channel and geometrical structure. This new biosensor requires standard integrated-circuit processing with relaxed fabrication requirements. The biosensor is composed of an accumulation-type, planar transistor surrounded by four gates, a backgate, front gate and two lateral gates, and it operates in the all-around-depletion mode. Consequently, adjustment of the four gates defines the dimensions and location of the conducting channel. It is shown that lithographically shaped channels of 400nm in width are reduced to effective widths of 25nm upon lateral-gate biasing. Device operation is demonstrated for protein-specific binding, and it is found that sensitive detection signals are recorded once the channel width is comparable with the dimensions of the protein. The device performance is discussed and analyzed with the help of three-dimensional electrostatic simulations.