A family of viruses distinguished by four characteristics: (1) genetic information is in the form of ribonucleic acid (RNA); (2) each single virus particle or virion (illus.c, arrow) possesses the enzyme reverse transcriptase; (3) virion morphology consists of two proteinaceous structures, a dense core (illus.a) and an envelope (illus.a) that surrounds the core; and (4) the viral reproductive cycle requires the synthesis of a deoxyribonucleic acid (DNA) copy of the viral genome and insertion of that DNA into the genome of the host cell.