Three genes coding for mouse tRNAPro have been isolated from a genomic library and characterized both structurally and functionally. Two of these (tPro52 and tPro53) code for the tRNA primer of reverse transcriptase of MuLV. The third one (tPro51) shows several differences (mutations and deletions) that probably prevent the folding of the matured transcript into the cloverleaf structure, and is therefore a pseudogene. This pseudogene gives rise to a RNA transcription product in vitro. tPro52 is clustered with a tRNALys gene and with a tRNAAla gene, which is strongly homologous to the rat identifier repeated sequence. tPro53 is clustered with a tRNAAsp and a tRNAGly gene. Other tRNA-hybridizing sequences are present in the lambda clones that contain tPro51 and tPro53.