Why do genes have introns? Recombination might add a new piece to the puzzle.

Determination of trace amounts of 5-methylcytosine in DNA by reverse-phase highperformance liquid chromatography. Anal. Biochem. 164, 164–169 6 Bird, A.P. (1995) Gene number, noise reduction and biological complexity. Trends Genet. 11, 94–100 7 Bird, A.P. and Wolffe, A.P. (1999) Methylationinduced repression – belts, braces, and chromatin. Cell 99, 451–454 8 Hung, M.S. et al. (1999) Drosophila proteins related to vertebrate DNA (5-cytosine) methyltransferases. Proc. Natl. Acad. Sci. U. S. A. 96, 11940–11945 9 Tweedie, S. et al. (1999) Vestiges of a DNA methylation system in Drosophila melanogaster? Nat. Genet. 23, 389–390 10 Wade, P.A. et al. (1999) Mi-2 complex couples DNA methylation to chromatin remodelling and histone deacetylation. Nat. Genet. 23, 62–66 11 Dong, A. et al. (2001) Structure of human DNMT2, an enigmatic DNA methyltransferase homolog that displays denaturant-resistant binding to DNA. Nucleic Acids Res. 29, 439–448 12 Okano, M. et al. (1998) Dnmt2 is not required for de novo and maintenance methylation of viral DNA in embryonic stem cells. Nucleic Acids Res. 26, 2536–2540 13 Lyko, F. et al. (2000) The putative Drosophila methyltransferase gene dDnmt2 is contained in a transposon-like element and is expressed specifically in ovaries. Mech. Dev. 95, 215–217 14 Lyko, F. et al. (2000) DNA methylation in Drosophila melanogaster. Nature 408, 538–540 15 Gowher, H. et al. (2000) DNA of Drosophila melanogaster contains 5-methylcytosine. EMBO J. 19, 6918–6923 16 Jaenisch, R. (1997) DNA methylation and imprinting: why bother? Trends Genet. 13, 323–329 17 Ramsahoye, B.H. et al. (2000) Non-CpG methylation is prevalent in embryonic stem cells and may be mediated by DNA methyltransferase 3a. Proc. Natl. Acad. Sci. U. S. A. 97, 5237–5242 18 Yoder, J.A. and Bestor, T.H. (1998) A candidate mammalian DNA methyltransferase related to pmt1p of fission yeast. Hum. Mol. Genet. 7, 279–284 19 Lyko, F. et al. (1999) Mammalian (cytosine-5) methyltransferases cause genomic DNA methylation and lethality in Drosophila. Nat. Genet. 23, 363–366 20 Roder, K. et al. (2000) Transcriptional repression by Drosophila methyl-CpG-binding proteins. Mol. Cell Biol. 20, 7401–7409 21 Ahringer, J. (2000) NuRD and SIN3 histone deacetylase complexes in development. Trends Genet. 16, 351–356 22 Larsson, J. et al. (1996) Mutations in the Drosophila melanogaster gene encoding S-adenosylmethionine synthetase suppress position-effect variegation. Genetics 143, 887–896 23 Larsson, J. and Rasmuson-Lestander, A. (1994) Molecular cloning of the S-adenosylmethionine synthetase gene in Drosophila melanogaster. FEBS Lett. 342, 329–333