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 or a derivative or mutant thereof, 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.