A current question of considerable interest to both the medical and nonmedical communities concerns the number of human transcription units (which, for the purposes of this paper, are “genes”) and proteins. Even with the recent announcement of the completion of the draft sequence of the human genome, it is still extremely difficult to predict the number of genes present in the genome. There are several methods for gene prediction, all involving computational tools. One way to approach this question, involving both computation and experiment, is to look at copies of fragments of messenger ribonucleic acid (mRNA) called expressed sequence tags (ESTs). The mRNA comes only from a gene being expressed, or translated, into RNA; by clustering mRNA fragments, we can try to reconstruct the expressed gene. While the final result is a very rough representation of the “true expressed transcript,” it is probably within 20% of the real number. Here, we review the issues involved in EST clustering and present an estimate of the total number of human genes. Our results to date indicate that there are some 70000 transcription units, with an average of 1.2 different transcripts per transcription unit. Thus, we estimate the total number of human proteins to be at least 85 000. The total number of proteins will be higher because of post-translational modification.
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