Mutations in genes important for the preservation of genome stability can increase the frequency of gene amplification, a process relevant to tumor development. To investigate whether telomerase, the enzyme deputed to telomere maintenance, also plays a role in gene amplification, we studied the amplification of the carbamyl-P-synthetase, aspartate transcarbamilase, dihydro-orotase ( CAD ) gene in immortalized embryonic fibroblasts derived from telomerase knockout mice (m TERC^−/− ) of the first and of the sixth generation. As expected, in 9 out of 10 N-(phosphonacetyl)- l -aspartate (PALA) resistant clones derived from wild-type cells, CAD was amplified; in contrast, in none of the 30 PALA resistant clones isolated from the three m TERC^−/− cell lines we could detect CAD amplification, indicating that, in the absence of telomerase activity, gene amplification is inhibited. The causal relationship between m TERC deficiency and lack of gene amplification was demonstrated by the restoration of CAD gene amplification in two of the three deficient cell lines transfected with m TERC . The lack of amplification in m TERC deficient cells could be related to a defect in the stabilization of the ends of the amplified chromosomes in the absence of telomerase, to a more general effect of telomerase in the regulation of gene expression, including genes involved in amplification, or to a possible interaction of the telomerase RNA with proteins involved in gene amplification.