We performed a comparative analysis on CHO cells with varying recombinant monoclonal antibody production rates (qAb) and investigated the regulation of MAb production. Two families of CHO cells each composed of one parental and two progeny cell lines, generated by stepwise increases in methotrexate (MTX) concentration, were studied. The MAb heavy chain (HC) and light (LC) gene copy numbers and mRNA levels were quantitated by Southern and northern blotting in midexponential growth phase. We observed 2- to 3-fold amplification in gene copy numbers in high producing cell lines when compared with parental cell lines. However, the mRNA levels in the high producers were 5- to 7-fold higher, correlating well with the 5- to 7-fold increase in qAb. These results were confirmed by real-time qPCR analysis. Our study reveals that the MTX-mediated MAb overexpression results from both an increase in the gene copy number and more efficient transcription of each gene copy. We also observed that the HC to LC gene copy ratio may affect the protein yield in MAb expression systems.