Description of data collection method and overview of results. (a) Diagram of expression tag capture. L1 elements are often naturally transcribed with non-L1 sequences at the 5' and/or 3' end. A 3' RACE adaptor/oligo(dT) primer and L1 specific primer can be used to capture expressed sequence from the 3' end. Similarly, a 5' RACE adaptor and L1 specific primer can capture 5' start sites that occur in non-L1 sequence. PCR is subsequently used to amplify the signal from the expressed tags. (b) Examples of 3' expression tags. Sequences start at the end of the L1 and terminate in 12 adenosines derived from the 3' RACE adaptor. (c) Histogram depicting the distribution of polyadenylation positions of 3' expression tags, relative to the end of the L1. (d) Histogram depicting distribution of 5' start site positions upstream of the 5' end of full-length L1 elements. Negative start sites occur in the 5' UTR downstream of the consensus 5' end of the element. Nt, nucleotides.