Catalogues comprehensively the isoelectric points (pIs) of proteins. Protein pI data were collected through scrutiny of the primary scientific literature. PIP-DB contains 5773 protein entries, each associated with either a single pI value or pI range. Each protein entry is linked to additional associated data: experimental data (stored within PIP-DB) and cross references to external data sources. Experimental data include, where available, temperature, method of analysis (IEF, cIEF, etc.), total measured molecular weight (MW), number of subunits, subunit MW, Enzyme Commission (EC) number, source organism and cellular and/or tissue location.