The TP53 gene expresses at least nine different mRNA variants (p53 isoform mRNAs), including the one encoding the canonical p53 tumor suppressor protein. We have developed scientific tools to specifically detect and quantify p53 isoform expression at mRNA level by nested RT-PCR (reverse transcription-polymerase chain reaction) and quantitative real-time RT-PCR (RT-qPCR using the TaqMan(®) chemistry). Here, we describe these two methods, while highlighting essential points with regard to the analysis of p53 isoform mRNA expression.