p51, a novel family member of human p53, is a recently identified candidate tumor suppressor gene mapped at chromosome 3q28. Like p53, p51 was found to activate p21Waf1/Cip1 and to induce apoptosis. Since the DNA loss at 3q is reported in several cancers including non-small cell lung cancer (NSCLC), we screened for mutations in p51A (TAp63gamma), an isoform of p51 with short C-terminal region, in 80 NSCLCs as well as 85 breast cancers by RT-PCR single strand conformation polymorphism (SSCP) analysis and DNA sequencing. In NSCLCs, p51 was expressed in most tumors at variable levels and we found three missense and one silent mutations: Gln31His (transactivation domain) in two tumors, Ala148Pro (DNA-binding domain) and Leu248Leu (DNA-binding domain). In the tumor with Ala148Pro or the silent mutation, only the mutant gene appeared to be expressed. The modified FASAY method to test the ability of yeast expressing p51A cDNA to grow in medium lacking histidine has revealed that Ala148Pro results in a loss of function, while Gln31His does not. In contrast to NSCLC, no mutation was observed in all 85 breast cancers by the similar method. Our results suggest that, because of infrequent mutation, p51 may not be a Knudson type tumor suppressor in most NSCLCs and breast cancers. Nevertheless, in at least a part of NSCLC, p51 may play a certain role in carcinogenesis in a tissue-specific manner.