The present invention is generally related to compositions, kits and methods for labeling nucleic acid molecules using reverse transcriptases, preferably multi-subunit reverse transcriptases such as ASLV reverse transcriptases. Specifically, the invention relates to methods, kits and compositions for to fluorescently labeling nucleic acid molecules during nucleic acid synthesis. The labeled nucleic acid molecules produced in accordance with the invention are particularly suited as labeled probes for nucleic acid detection and diagnostics.