We investigated the mechanisms responsible for paclitaxel resistance in HME-1 cells (human mammary epithelial cells immortalized with hTERT). These cells were exposed to paclitaxel (10 pM for 7 days) and 20 cellular surviving populations (PSP) were obtained. PSP demonstrated high levels of resistance to paclitaxel cytotoxicity as compared with HME-1 cells. Activation of mdr-1 gene expression was observed in 2 PSP. Protein expression analysis using a C-terminal targeted antibody showed that 13 PSP were negative for p21/WAF1 expression after ionizing radiation (6 Gy) or doxorubicin (100 nM) treatment. Sequencing of the 3 exons of the CDKN1A gene revealed that 13 PSP contained a point mutation in exon 2. This mutation consisted in a T insertion at codon 104 leading to a premature STOP codon appearance. Mismatch amplification mutation assay and RFLP-PCR confirmed the presence of the mutation in 16 PSP. Western blot using an N-terminal targeted antibody demonstrated that the C-terminal-truncated p21/WAF1 protein (14 kDa) was indeed expressed in the 13 PSP. Our data suggest that p21/WAF1 inactivation may confer a strong resistance to paclitaxel in noncancerous breast epithelial cells harboring a p21/WAF1 mutant.