Abstract: A method for detecting the presence of a gene responsible for a pathological state; or the pathological state itself in a patient comprising exposing at least one hair from the patient to fibre X-ray diffraction and detecting changes in the ultrastructure of the hair. An instrument (1) for detecting the presence of a gene responsible for a pathological state; or de pathological state itself, using a hair sample, comprising: an X-ray source (2) producing a beam of X-ray radiation; a sample stage for positioning said hair (3) sample within said beam; a detector to detect the scanering of said X-ray beam caused by said hair sample; and a display means (9) associated with said detector for displaying the output thereof (9a, 9b, 9c and 9d); whereby patterns of output related to the presence of said gene or said pathological state are displayed for interpretation.