Transgene copy number and stability study in recombinant PT1-CHO cell lines. a Copy number of integrated transgenes was determined by quantitative PCR (qPCR) using primers on the light chain cDNA of PT1 construct at an early passage (P5). PT1-7 was used as a calibrator with a known copy number of one which was previously determined by Southern blot hybridization (data not shown). Samples were measured in triplicates. PT1-30 and PT1-55 revealed more than one copy of the transgene. Data represent means and standard error of the means (SEM) of n = 3 measurements. b A stability study was initiated with a relative value of 1 as a starting titer value. During the study, all clones showed instability to various degrees. In the most unstable clone PT1-1, a drop of productivity to 0.2 was measured whereas PT1-7 showed a loss of titer to only 0.8.