One of the most important criteria for successful generation of a therapeutic protein from a recombinant cell is to obtain a cell line that maintains stability of production. If this is not achieved it can generate problems for process yields, effective use of time and money, and for regulatory approval of products. However, selection of a cell line that sustains stability of production over the required time period may be difficult to achieve during development of a therapeutic protein. There are several studies in the literature that have reported on the instability of protein production from recombinant cell lines. The causes of instability of production are varied and, in many cases, the exact molecular mechanisms are unknown. The production of proteins by cells is modulated by molecular events at levels ranging from transcription, posttranscriptional processing, translation, posttranslational processing, to secretion. There is potential for regulation of stability of protein production at many or all of these stages. In this study we review published information on stability of protein production for three industrially important cell lines: hybridoma, Chinese hamster ovary (CHO), and nonsecreting (NSO) myeloma cell lines. We highlight the most likely molecular loci at which instability may be engendered and indicate other areas of protein production that may affect stability from mammalian cells. We also outline approaches that could help to overcome the problems associated with unpredictable expression levels and maximized production, and indicate the consequences these might have for stability of production. (C) 2003 Wiley Periodicals Inc.