Fig. 4. Mature snRNPs do not show a measurable export from the nucleus to the cytoplasm. (A) HeLa cells stably expressing YFP-SmB were bleached repeatedly using a region of interest including a region of the cytoplasm of one cell and a region of the nucleus of its neighbour (rectangle in first image). The cells were imaged following each bleach event. In the cell undergoing bleaching of the nucleus, the entire nucleus was rapidly depleted of YFP-SmB. By contrast, the cell undergoing cytoplasmic bleaching lost virtually none of its nuclear signal. (B) Intensity of YFP-SmB signal in the nucleus (outside the bleached region) for the two bleached cells against time.