The technology of gene knockout and transgenic mice has allowed the study of the role of genes and their proteins in animal physiology and metabolism. However, these techniques have often been found to be limited in that some genetic manipulations of mice led either to a fatal phenotype or to compensations that mask the loss of function of the target protein. The experimentation on neurons from transgenic mice is particularly critical in the study of key proteins that may be involved in neurodegeneration. The cell fusion technique has been implemented as a novel way to generate cell lines from prion protein knockout mice. Fusion between neonatal mouse neurons and a neuroblastoma cell line have led to a Prnp°/° cell line that facilitates the study of the knockout phenotype. These cells are readily transfectable and allowed us to study the expression of prion protein mutants on a PrP-knockout background. Using this cell line we have examined the effect of PrP mutations reported to alter PrPc to a transmembrane form. Our results suggest that these mutations do not create transmembrane forms of the protein, but block normal transport of PrP to the cell membrane.