MicroRNAs (miRNAs) are short non-coding RNAs that play a role in the control of gene expression. In the March 7 Science, Lee Lim and colleagues at the Massachusetts Institute of Technology, report attempts to predict the total number of miRNAs in the human genome (Science, 299:1540, March 7, 2003).

Lim et al. used a computational program called MiRscan to identify miRNA genes in vertebrate genomes by evaluating conserved stem loop structures. Around 15,000 potential stem loop structures were found in the human genome. MiRscan identified a set of 188 human loci with high scores, including 81 (i.e. 74%) out of 109 known human miRNAs. They predict that the upper limit for the number of human miRNAs is 255 genes, suggesting that there are only another 40 or so miRNA genes to be found. When all these have been tracked down, the challenge will be to work...

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