Tumour suppressor p53, the most frequently altered gene among human cancers, is known to function by modifying gene transcription. Transcriptional regulatory activity of p53 has been established using transfected cell culture models and in vitro systems. Here, we report the characterisation of transcriptional function of an endogenous mutant p53 (p53(273.His)) in MDA-468 human breast cancer cells, in terms of DNA-binding and transactivation assays. We have detected specific complexes of p53(273.His) with a high affinity consensus sequences (CON) and with Fragment A (FRA) sequences, two of the known p53 DNA binding sites. Furthermore, these sequences mediated p53-specific transcriptional modulation of a reporter gene. While CON conferred 10 fold transactivation of transcription, FRA, on the other hand, suppressed transcription to a similar extent in MDA-468 cells. These experiments indicate a sequence-dependent novel transcriptional regulatory function for endogenous p53(273.His) in MDA-468 cells.