One little-explored mechanism whereby cell signals could influence the transcription of target genes would be by regulating the permeability of the nuclear envelope to transcription factors and other nuclear proteins. O’Brien et al. compared the distribution in SKHep1 liver cells of a 27-kD photoactivatable green fluorescent protein construct (PA-GFP) to that of a DsRed construct targeted to the cytoplasm (DsRed-NES) and found that PA-GFP locally excited by means of two-photon excitation did not appear to cross the nuclear envelope. PA-GFP activated in the nucleus and viewed for at least 5 minutes diffused throughout the nucleus but did not enter the cytoplasm, and PA-GFP excited within the cytoplasm did not enter the nucleus. Exposure to vasopressin, or other hormones that activate the Gq signaling pathway (and thereby elicit an increase in cytosolic Ca2+ concentration), increased permeability of the nuclear envelope to PA-GFP unidirectionally, so that PA-GFP activated in the cytoplasm spread into the nucleus. Increasing cytosolic calcium concentration by inhibiting the sarcoplasmic and endoplasmic reticulum Ca2+-ATPase also increased unidirectional nuclear membrane permeability, an effect that was blocked by a Ca2+ chelator. Moreover, localized photorelease of caged Ca2+ in the cytoplasm near the nuclear envelope led to a localized increase in unidirectional nuclear envelope permeability. Thus, the authors conclude that one mechanism whereby hormones that activate intracellular Ca2+ signaling pathways may modulate transcription of target genes is through the Ca2+-dependent regulation of nuclear envelope permeability. These studies raise the question of how the PA-GFP activated in the nucleus got there. The authors suggest that this may have occurred during mitosis (when the nuclear envelope breaks down) or in response to previous stimulation by growth factors.