The 5' UTRs of mouse L1s contain a variable number of GC-rich tandemly repeated monomers of around 200 bp, followed by a short non-monomeric region. Human 5’ UTRs are ~900 bp in length and do not contain repeated motifs. All families of human L1s harbor in their most 5’ extremity a binding motif for the transcription factor YY1. Younger families also have two binding sites for SOX-family transcription factors, and both YY1 and SOX sites were shown to be required for human L1 transcription initiation and activation. Both mouse and human 5’ UTRs also contain a weak antisense promoter of unknown function.