Microarrays and molecular markers for tumor classification.

Human cancers have traditionally been classified according to their tissue of origin, histological characteristics and, to some extent, molecular markers. Clinical studies have associated different tumor classes with differences in prognosis and in response to therapy. Measurement of the expression of thousands of genes in hundreds of cancer specimens has begun to reveal novel molecularly defined subclasses of tumor; some of these classes appear to predict clinical behavior, while others may define tumor types that are ripe for directed development of therapeutics. Unfortunately, at present, differences between studies of similar tumor types can be as striking as their similarities.