1520 Nucleic Acids Research, 2006, Vol. 34, No. 5 The heterogeneity associated with L1 splicing, and its potential to negatively impact both the L1 life cycle and host genes, makes it seem unlikely that most of the splicing observed evolved for a speciﬁc purpose. We favor the hypothesis that the A-richness of the L1 coding regions may contribute to the ability of L1 RNAs to splice. Thus, the A-richness may be the cause of multiple forms of silencing of, and by, L1 sequences (17,18,57). SUPPLEMENTARY DATA Supplementary Data are available at NAR Online. ACKNOWLEDGEMENTS We would like to thank Dr A. Engel and the members of the Deininger laboratory for helpful discussions. This work was supported by grants from Department of Defense Breast Cancer Research Program, DAMD17-02-1-0597 (V.P.B.), the National Institutes of Health, R01GM45668 (P.L.D), National Science Foundation, EPS-0346411 (P.L.D), and the State of Louisiana Board of Regents Support Fund (P.L.D). 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