<p>Psoriasis is a chronic skin disorder which affects between 1 and 3 per cent of the population. The condition is characterised by well defined red, scaly lesions which occur symmetrically, frequently on the elbows, knees, lower legs and scalp. Any part of the skin can be affected including the limb flexures, nails and less commonly the face. Although the symptoms and pathology of psoriasis are well documented, the causes are not.</p>