</ref> the prevailing approach is to classify it into three clinical sub-types called Usher I, II and III in order of decreasing severity of deafness. Although it was previously believed that there was an Usher syndrome type IV, researchers at the University of Iowa recently confirmed that there is no USH type IV. As described below, these clinical subtypes may be further subdivided by the particular gene mutated; people with Usher I and II may have any one of six and three genes mutated, respectively, whereas only one gene has been associated with Usher III. The function of these genes is still poorly understood.