CXCL12 and viral infectious disease: After two weeks in culture, the extent of viral infection was assayed by measuring the reverse transcriptase activity in the supernatant, as a measure of viral replication in the culture [32] In six separate experiments, Peptide 3 (100 μM) had no effect on virus replication following HIV exposure (Fig 3a), while SDF1α inhibited reverse transcriptase activity by an average of 75%.