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We identified a temperature-sensitive allele of small bristles (sbr), the Drosophila ortholog of human TAP/NXF-1 and yeast Mex67, in a screen for mutants defective in mRNA export. We show that sbr is essential for the nuclear export of all mRNAs tested in a wide range of tissues and times in development. High resolution and sensitive in situ hybridization detect the rapid accumulation of individual mRNA species in sbr mutant nuclei in particles that are distinct from nascent transcript foci and resemble wild-type export intermediates. The particles become more numerous and intense with increasing time at the restrictive temperature and are exported very rapidly after shifting back to the permissive temperature. The mRNA export block is not due indirectly to a defect in splicing, nuclear protein import, or aberrant nuclear ultrastructure, suggesting that in sbr mutants, mRNA is competent for export but fails to dock or …