There's growing conviction that gene arrays are less useful than first imagined for expression studies and diagnostics--and that directly reading protein expression will more likely provide an accurate picture of biological status-health, disease, and pharmaceutical response. Thus, a number of companies have started up to create, on the analogy of gene arrays, protein arrays, to allow the simultaneous and quantitative testing of large numbers of proteins-potentially thousands.But the task is easier said than done. First, there are too few proteins known to allow testing for worthwhile expression patterns; and, second, because of the delicacy and variety of proteins, surface chemistry and other problems have historically made arrays impractical. None of this daunts the start-ups who, opportunistic, are looking to exploit the markets they think they can get to first: clinical diagnostics and pharmaceutical clinical development programs.