Background Investigations have shown animal cell cultures’ performance, in terms of cell proliferation and production of recombinant protein, are negatively affected by both lactate’s concentration and its specific production rate. In a previous work, we determined that lactate production was caused by pyruvate accumulation due to its high synthesis rate in the glycolitic pathway and limited consumption in the TCA cycle, which leads to lactate production [1]. In this work, we use the ΔL/ΔHexose ratio in order to characterize the cells metabolic state. This ratio describes the lactate production rate vs. hexose consumption. Low ΔL/ΔHexose ratios indicate efficient metabolic states where carbons consumed are mainly used to support cell growth, protein synthesis or energy metabolism. Cell engineering has been previously used to improve cultures’ performance by changing the expression of genes involved in metabolism and apoptosis, focusing on the modification of only one gene at the time. These works showed that after overexpression of genes such as fructose transporter (Slc2a5) and yeast’s pyruvate carboxylase (PYC) cells are able to achieve higher cell densities and lower lactate production than wild-type cells under the same culture conditions [2-4]. In this work we aim at introducing multiple changes in the cells’ genome in order to obtain an engineered cell line with reduced lactate production and enhanced energy metabolism, which is capable of achieving higher cell densities and with a longer lifespan. We propose to control both, carbon uptake and its use by the TCA cycle. Cells were transfected with the fructose transporter gene (Slc2a5) and pyruvate carboxylase gene (PYC). Metabolic flux redistribution was studied through metabolic flux analysis, comparing engineered cells and wildtype under normal culture conditions.