Drug targets estimated number

It has been estimated that 20-30 per cent of proteins encoded by the human genome are membrane proteins. Clearly, this makes membrane proteins a vital class to understand, and furthermore more than 70 per cent of drug targets are membrane proteins. A significant problem in mass spectrometry is that a far smaller number of membrane proteins are identified... 

It is estimated that transporters account for about 50% of drug targets. However, their modes of (selective) transport are only poorly understood. This is due to difficulties in membrane protein purification, expression, and crystallization (Caffrey, 2003), which is still in its childhood. As a consequence there exists a striking disproportion between the number of entries of resolved structures of soluble proteins and membrane proteins in the protein databank (PDB, http //www.pdb.org/). To date, only about 2% (1462 by Sept 2011) of the structures are from transmembrane proteins (75594 structures in total). Out of these there are only 302 unique structures (proteins of same type but from different spjecies are included) (Irvine). Moreover, a significant number of the membrane protein structures determined are at relatively low resolutions (Lindahl Sansom, 2008). 

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