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The CHO-K1 cell line is the most common expression system for therapeutic proteins in the pharmaceutical industry. Due to the nature of economics, the cell lines and the vector design are subject to constant change to increase product quality and quantity. During the cultivation, the production cell lines are susceptible to decreasing productivity over time. Often the loss of production can be associated with a reduction of copy number and the silencing of transgenes. During cell line development, the most promising cell lines are cultivated in large batch culture. Consequently, the loss of a stable production cell line can be very cost-intensive. For this reason I developed different strategies to avoid a reduced productivity. Instability of production cell lines can be predicted by the degree of CpG methylation of the driving promoter. Considering that the DNA methylation is at the end of an epigenetic cascade and associated with the maintenance of the repressive state, I investigated the upstream signals of histone modifications with the assumption to obtain a higher predictive power of production instability. For this reason I performed a chromatin immunoprecipitation of the histone modifications H3K9me3 and H3K27me3 as repressive signals and H3ac as well as H3K4me3 as active marks. The accumulations of those signals were measured close to the hCMV-MIE at the beginning of the cultivation and were then compared with the loss of productivity over two month. I found that the degree of the H3 acetylation (H3ac) correlated best with the production stability. Furthermore I was able to identify an H3ac threshold to exclude most of the unstable producers. In the second project I aimed to improve the vector design by considering epigenetic mechanisms. To this end I designed on the one hand a target-oriented histone acetyltransferase to enforce an open and active chromatin status at the transgene. On the other hand I point-mutated methylation-susceptible CpGs within the hCMV-MIE to impede the maintenance of inactive heterochromatin formation. Remarkably, the C to G mutation located 179 bp upstream of transcription start site resulted in very stable antibody producing cell lines. In addition, the examination of cell pools expressing eGFP showed that G-179 promoter variants were less prone to a general methylation and gene amplification, which illustrates the dominating effect in epigenetic mechanisms of one single CpG. The last project was performed to localize stable integration sites within the CHO-K1 genome. In so doing I could show that the transfection leads predominantly to integration into inactive regions. Furthermore I identified promising integration sites with a high potential to induce stable expression. However, those results are preliminary and must be viewed with caution. Further examination needs to be done to confirm these results. Considering the results of all three projects, I propose that the interplay of metabolic burden and selection pressure at an early time point of cultivation plays an important role in cell line development. Small alterations of selection pressure can lead to a decisive change of cell properties. Therefore, stable cells are less susceptible than weak producers. The increase of selection pressure leads to compensatory effect by gene amplification in the instable cell lines. The resulting adjustment of productivity masks the truly stable cells, which precludes the selection of the right cell lines. For this reason the selection pressure, the copy number as well as the growth rate should be considered to minimize repressive effects.