Protein p53 as a tumor suppressor plays a central role in the cell cycle control and tumorigenesis. It was recently discovered that the gene p53 encodes nine different protein isoforms (p53, p53β, p53γ, 133p53, 133p53β, 133p53γ, 40p53, 40p53β and 40p53γ) due to alternative splicing and usage of the alternative promoter in intron 4. The expression of isoforms is tissue-specific, and their biological function is not fully understood. p53 gene has two relatives of major structural and functional similarities, p63 and p73. They also have a number of isoforms, which make p53 activity more complex. Luciferase assay was used to detect transcriptional activity of isoforms, p73β, p53, p53β, p53γ, 133p53, 133p53β, 133p53γ and 40p53 that were trasfected into H1299 cells, which lack expression of wildtype p53 protein. Transcriptional activity of each isoform was detected. p73β and p53β isoforms show the greatest degree of ranscriptional activity, compared to full-length p53. p53γ and 40p53 show a significant level of transcriptional activity, whereas 133p53, 133p53β and 133p53γ have extremely low levels of transcription activity. Western blot method has shown the expression of all isoforms at the protein level analysis.