The present invention relates to the production of proteins in host cells, and more particularly to host cells containing multiple integrated copies of an integrating vector. Suitable integrating vectors for use in the present invention include retrovirus vectors, lentivirus vectors, transposon vectors, and adeno-associated virus vectors. Methods are provided in which the host cells are prepared by using the integrating vectors at a high multiplicity of infection. The host cells are useful for producing pharmaceutical proteins, variants of proteins for use in screening assays, and for direct use in high throughput screening.