Human T-lymphotropic retroviruses. 
The first human retroviruses have been discovered during the past six years.  They cause two diseases which involve disturbances of the growth of the T4 lymphocyte, a remarkably specific target cell type.  This cell, which is central to the regulation of the immune system, is induced by human T-lymphotropic virus type I (HTLV-I) to excessive proliferation (leukaemia) and by HTLV-III to premature death (acquired immune deficiency syndrome, AIDS).  Both also seem to be indirectly involved in several other disorders.  The genetic structures of these retroviruses and the mechanisms by which they usurp host-cell functions are novel among retroviruses.