In male, the prostate cancer (PCa) is one of the most frequent neoplasias and the second cause of cancer deaths worldwide. In 2015, more than 6000 men died in Mexico due to this disease. In this regard, prostate cancer associated gene 3 (PCA3) has become an interesting target in PCa as is found highly overexpressed. Moreover, TAAA tandem repeats have been suggested to be associated with the regulation of PCA3 expression and, in turn, to be related with the development of the disease. The aim of the study was to understand thegenetic basis of the disease in search for a better diagnosis. Expression levels of PCA3 gene were analysed in tissue of 13 patients diagnosed with PCa and six patients diagnosed with a benign prostatic disease (BPD). The absolute expression of PCA3 was quantified by real-time PCR. Genotype for TAAA tandem repeats was measured using automatic sequencing and the results were analysed to determine whether an association existed between them. We identified three alleles: 4, 5, 6 and four genotypes: 4/5, 5/5, 5/6, 6/6. Our analysis identified amutation in the nucleotide 76764237 of the PCA3 gene that generates an extra TAAA tandem repeat. The nucleotide mutation is present in 61.53% of PCa and 66.66% of BPD patients. Our study revealed the presence of a mutation in the PCA3 gene that generates an extra TAAA tandem. We observed no association between the absolute expression of PCA3 messenger and the number of TAAA repetitions.