Protein expression using a BAC as vector backbone is transgene copy-number dependent and it is stable over time. A) Correlation between protein production and integrated BAC transgene copies. Analysis of 12 single clones showed a copy number range from 1 to 55 for the Rosa26BAC CAGGS Fc vector with a Fc yield of 5.5 to 30 pg/cell/day. The protein production was directly proportional to the BAC transgene copy number. B) Protein production is stably maintained during culture passaging. The yield of Fc protein was measured at passage 1 and passage 30 in 6 subclones isolated from the Rosa26BAC CAGGS Fc cultures. No obvious differences were found. Error bars represent the standard deviation.