The knockdown of genes that are over-expressed in cancer, and function in tumor onset and/or progression, is an attractive tool to impair the growth of tumor cells. Synthetic nucleic acids such as antisense oligodeoxynucleotides (AS-ODNs) or small-interfering RNAs (siRNAs) were applied against different tumor-associated transcripts, including the human telomerase reverse transcriptase (hTERT), to inhibit the proliferation of tumor cells and to sensitize them against chemotherapeutic (CT) agents. The efficacy of nucleic acid-based inhibitors was evaluated in vitro by determining the extent of down-regulation of the respective target mRNA and protein expression as well as by extensively investigating growth properties (e.g., viability, proliferation, apoptosis, and cell-cycle distribution) of the affected tumor cells. Methods for a successful down-regulation of hTERT and for the quantitative determination of resulting effects on cellular growth were described herein.