1. A stably transfected immortalized human cell line prepared by transfecting an immortalized human host cell line under serum-free conditions with a transfection vector comprising a nucleic acid sequence comprising a gene encoding a human target protein, a promoter, and a bovine growth hormone polyadenylation (polyA) signal, the promoter and polyA signal linked to the 5′ and 3 ′ end of the gene encoding the human target protein, respectively, and an origin of replication, resulting in a stably transfected immortalized human cell line.