Objective Mammalian cells as the main host for production of human proteins are incapable of complete gamma-carboxylation of over-expressed Vitamin K Dependent (VKD) proteins. The Drosophila gamma-glutamyl carboxylase (D gamma C) has been shown to be more efficient than its human counterpart in gamma-carboxylation of human substrates, in vitro. Considering the Drosophila gamma-carboxylase (D gamma C) efficiency, in comparison with its human counterpart, for recognition and gamma-carboxylation of a human substrate in vitro, we were determined to study the effect of the D gamma C on the hFIX expression in a mammalian cell line. With this aim, we examined co-expression of the D gamma C with the hFIX, in a human cell line. Results While the co-expression of a complete D gamma C cDNA reduced the hFIX expression, a truncated form of D gamma C could improve both the expression level (up to 1211 ng/10(6) cells/ml on the 4th day of post-transfection) and carboxylation of the expressed hFIX, significantly (p < 0.009). Conclusions Our findings provided evidences for potential of a partial fragment of the D gamma C for improvement of the gamma-carboxylation of a human substrate in a mammalian cell. Our experimental data, in accordance with in silico analysis suggested that the D gamma C C-terminal fragment, with the advantage of a Kozak-like element has the potential of being expressed as a separate internal translation unit, to generate a peptide with appropriate gamma-carboxylase activity.