The structural integrity of the p53 gene in a human thyroid-medullary-carcinoma-derived cell line has been studied. Analysis of high-molecular-weight DNA showed that the p53 locus is severely rearranged. PCR and single-strand conformation polymorphism analysis revealed that a large portion of the 5' end of the p53 gene is lost, while a region encompassing exons 8 and 9 is rearranged. As a consequence, no virtual expression of a p53-specific transcript is detected in mRNA from the medullary-carcinoma cell line. The absence of a p53 protein prompted us to analyze the biological effect of exogenous expression of this tumor-suppressor gene on cell growth and viability, introducing retroviral constructs carrying full-length human wild-type p53 cDNA. Contrary to what has been described for other cell types, including most thyroid-carcinoma cell lines of follicular origin, these experiments allowed us to establish clonal-cell populations which constitutively express p53. Cytometric analysis revealed G1-specific cell-cycle arrest, responsible for growth retardation in the transfected clones when compared with the parental cell line. However, medullary-thyroid-carcinoma cells expressing p53 are able to partially overcome the G1 block and progress through the cell cycle. In the search of the mechanism(s) involved in these processes, we describe the interaction of p53 with specific p21WAF1/Cip1 promoter sequences by gel-retardation assays.