Retrovirus infection proceeds by attachment of the envelope glycoprotein to a cell surface receptor, followed by fusion of the viral and cellular membranes. Once in the cell, the viral enzymes and structural proteins form a replication complex that converts the single-stranded viral genomic RNA into a double-stranded DNA, which is then integrated into the host cell chromosome. For HTLV-1, these events are not well characterized. We have developed cell culture systems, infectious molecular clones, and viral vectors that can be used to characterize the mechanisms of HTLV-1 infection and replication. Infection with cell-free HTLV-1 virions is orders of magnitude less efficient compared with other retroviruses. This inefficiency is the result of a block in the replication process after the virion is bound to the cell surface. We are determining whether this block is conferred by the viral replication enzymes, results from the actions of cellular restriction factors, reflects the need for cell-cell contact, or is caused by a combination of these factors.