The first membrane spanning region of the lamin B receptor is sufficient for sorting to the inner nuclear membrane

The nuclear envelope staining pattern displayed in each figure is typical for the majority of cells (70-90%) expressing the transfected gene. A minority of the expressing cells in each experiment (10-20%) produced unusually high levels of the transfected protein resulting in an obscuring of the exclusive rim stain. In these cases the protein appeared in the PER, Golgi complex, and in some cases on the cell surface. Overexpression and the resulting loss in the fidelity of localization has been discussed by others using similar transfection systems (Li and Bingham, 1991; Munro, 1991) . This pattern correlates with extremely high levels of expression and is therefore most apparent at late times after transfection (48-72 h). Thus, we restricted our analysis to 24--40 h after transfection.