About 5000 (25%) of the ~20400 human protein-coding genes currently lack any 
experimental evidence at the protein level. For many others, there is only 
little information relative to their abundance, distribution, subcellular 
localization, interactions, or cellular functions. The aim of the HUPO Human 
Proteome Project (HPP, www.thehpp.org ) is to collect this information for every 
human protein. HPP is based on three major pillars: mass spectrometry (MS), 
antibody/affinity capture reagents (Ab), and bioinformatics-driven knowledge 
base (KB). To meet this objective, the Chromosome-Centric Human Proteome Project 
(C-HPP) proposes to build this catalog chromosome-by-chromosome ( www.c-hpp.org 
) by focusing primarily on proteins that currently lack MS evidence or Ab 
detection. These are termed "missing proteins" by the HPP consortium. The lack 
of observation of a protein can be due to various factors including incorrect 
and incomplete gene annotation, low or restricted expression, or instability. 
neXtProt ( www.nextprot.org ) is a new web-based knowledge platform specific for 
human proteins that aims to complement UniProtKB/Swiss-Prot ( www.uniprot.org ) 
with detailed information obtained from carefully selected high-throughput 
experiments on genomic variation, post-translational modifications, as well as 
protein expression in tissues and cells. This article describes how neXtProt 
contributes to prioritize C-HPP efforts and integrates C-HPP results with other 
research efforts to create a complete human proteome catalog.