Abstract
Eucaryotic messenger RNA precursors are processed in nuclear ribonucleoprotein particles (hnRNP). Here recent work on the structure of hnRNP is reviewed, with emphasis on function. Detailed analysis of a specific case, the altered assembly of hnRNP in heat-shocked Drosophila and mammalian cells, leads to a general hypothesis linking hnRNP structure and messenger RNA processing.