Although the division of medicine into specialties according to different systems is convenient, it is also artificial: the different systems interact and many diseases overlap both in their pathological features as well as their clinical expression. Many examples of such interactions are seen in the connective tissue disorders, where rheumatological and dermatological manifestations may be prominent features. In some of them the skin rash may be a diagnostic marker (e.g., systemic lupus erythematosus, dermatomyositis). Joint involvement can also be found in "primary" skin disorders such as psoriasis; certain infections can produce both skin and joint manifestations including a number of fairly common viral disorders as well as Lyme borelliosis and the acquired immune deficiency syndrome (A.I.D.S.) The skin may also be the major target of toxicity from a number of drugs, particularly those that are used in the management of rheumatic disorders.