Diagram of possible scenarios for the origin of TRM. Non-homologous recombination between ancient IgH and TCR loci gave rise to a hybrid locus. A duplication event of part of the hybrid locus presumably created and additional set of V, D and J gene segments. Subsequently two possible alternatives are given for the origin of TRMVj. Left panel: A V(D)J recombination in germ cells followed by deletion of the intron located within the sequence encoding the leader peptide. Right panel: A V(D)J recombination event occurs in somatic cells, followed by transcription and splicing, resulting in a complete mRNA transcript. This mRNA was reverse transcribed to DNA by a reverse transcriptase and it was inserted back into the genome by homologous recombination. V, D and J gene segments and C regions are color-coded as in previous figures. TRMV and TRMVj are distinguished as red or yellow color, respectively. The exon encoding leader peptide is indicated with an L.