Nuclear import and export signals can be hard to detect in protein sequences because of their small size and lack of coherent motifs. In this issue, Rhee et al present a genetic method for detecting these signals based on a functional assay in yeast cells. First they modified the bacterial LexA protein, to abolish its intrinsic nuclear targeting activity, and then fused it to a sequence the yeast GAL4 activation domain, and the protein to be tested. If the cell contains a functional nuclear import signal, the fusion protein will enter the cell nucleus, where and activate the expression of β-galactosidase and a histidine biosynthesis gene. To assay for nuclear export, they made another construct in which fusion to proteins that contain a nuclear export signal will re-direct the protein into the cytoplasm, reducing β-galactosidase expression and growth on histidine deficient media (see p. 433).