The structurally related small (<5 kD) polypetides, namely thymosins beta-4 and beta-10, were originally defined in the rat immune system. Previously it was shown that both the beta-4 and beta-10 genes are constitutively expressed at higher levels in neoplastic human kidney. Also, it was shown that human embryonic kidney contained more of these proteins than the adult tis sue. The present study used a human thymosin beta-10 cDNA to examine the possibility that overexpression of the beta-10 mRNA in renal cell carcinoma was due to gene amplification. Southern blot analysis of genomic DNA extracted from normal and neoplastic tissue indicated no amplification of the thymosin beta-10 gene in RCC. No amplification or rearrangements were found in the human RAR-αgene in normal versus RCC tissue. Decreased expression of both the thymosin beta-4 and beta-10 proteins in the normal adult human kidney was found to be derived from a corresponding decrease in levels of the cognate mRNAs. These findings suggest that the thymosin beta-10 gene is deregulated in renal cell carcinoma.