  While the varied symptomatology of SLI suggests that no unified  theory of its etiology might be forthcoming, the cause of the  disorder is comparatively well understood in the case of one subtype, involving a severe disruption of morphosyntax (i.e., the rules  governing the formation of words from smaller semantic units, or  morphemes). This subtype, seen in about half the members of a large,  three-generation English family, the KE's, and in another, unrelated  individual, has been traced to a specific genetic mutation, the  function of which is actively under investigation.