We have generated a molecular description of the loci at which stability/instability of expression of a monoclonal antibody (MAb) (anti-CD38) occurs within the GS-NSO expression system. Critically, these data show that, in the absence of changes to copy number for the recombinant gene sequences, all cell lines examined exhibit a progressive loss (instability) in expression of mRNA during prolonged culture. However, not all cell lines express instability at the level of MAb protein production. The molecular distinction between stable and unstable production at the protein level is a reflection of the cellular amount of recombinant mRNA encoding MAb. Our data indicate a threshold level, a putative saturation point for utilisation of mRNA in translational/secretory events, that defines stability or instability of protein production. Above this level of recombinant mRNA expression, cell lines are stable, whereas below this level cell lines will show instability of protein production. Our studies indicate that absolute levels of expression of recombinant mRNA encoding for MAb in the GS-NSO expression system offer a potential predictive indicator for the selection of stable cell lines for scale-up. These studies identify molecular facets of host cell biology of generic interest for gene regulation and expression and define techniques and approaches for enhancement of recombinant protein expression and process development. (C) 2004 Wiley Periodicals, Inc.