Nuclear import of HIV-1 IN is importin α-dependent. W303 and in srp1-31 yeast cells were transformed with plasmid bearing the full length of the IN fused to GFP (pYES2yEGFP-IN for the construction of the plasmid see Methods). Following transformation using the lithium acetate method, the fusion protein GFP-IN was expressed in the yeast cells, as described in Methods. GFP fluorescence (green) and DAPI (blue) were observed under confocal microscope following growth of W303 yeast cells at 25°C or at 37°C, or of srp1-31 yeast cells at 25°C or at the non-permissive temperature, 37°C. Bar 5 μm.