<!------------Discussion----------->
If the 20 amino acids were used equally, each would occur at a level 
of 5% in every protein (excluding selenocysteine, whose total 
occurrence numbers a few dozen).
Of course, the amino acid composition of proteins genome-wide is actually quite 
different, with some residues in excess and others consistently depleted. 
For example, leucine has an average occurrence of 9.7%, over seven times the 
abundance of tryptophan at 1.3%. 
Furthermore, individual protein composition can differ markedly from these averages 
due to a variety of structural and functional reasons, such as long unstable runs 
of a single amino acid or the periodic residues in collagen helices. Higher ambient 
GC content in a patch of chromosome can also influence the amino acid 
content at weakly-conserved residues, in part from fixation of CpG hotspot
mutations.
<P>
The Protein Browser displays two histograms
showing amino acid frequencies and anomalies within the selected protein,
arrayed according to abundance. These anomalies can contain useful clues to
subcellular location, protein structure, function, and mutational
propensity. The distribution of anomalies along the protein primary
sequence is shown in the AA Anomalies track.
<P>
Compositional anomalies in general have no ready explanations. Indeed, a
certain number of departures from mean composition are expected for
purely statistical reasons. Anomalies may reflect a bulk physical attribute, such
as the need for many charged residues, or avoided residues (cysteines
stabilizing misfolding through disulfides). Anomalies may arise from
internal sequence repeats, such as runs of a single amino acid, or longer-period  
structural repeats, with every third residue being glycine to pack the
triple helix of collagen. At longer scales, multiply-repeated
domains can skew overall composition. Thus, compositional anomalies suggest
unusual overall amino acid abundances without necessarily suggesting an
explanation.
<P>
Tables of amino acid usage are available elsewhere for other species 
<A HREF="http://www.kazusa.or.jp/codon/" TARGET=_blank>by codon</A> and for
<A HREF="http://www.expasy.ch/sprot/relnotes/relstat.html"
TARGET=_blank>UniProtKB</A> as a 
whole. 
<P>
<B>Amino acid abundances based on the pepResDist table (July 2003 human assembly)</B>
<BR>
<BR>
<CENTER>
<IMG height=373 width=500 src="aaAnom.jpg">
</CENTER>
