In the course of studying the ST2 gene, which was initially found to be expressed specifically at the C&G, transitional state in BALBlc-3T3 cells and was one of the primary response genes, we found another ST2-related mRNA, designated as STZL, in serum-stimulated BALB/c-3T3 cells in the presence of cycloheximide. Nucleotide sequence analysis of the cloned STZL cDNA revealed that it had an open reading frame encoding a polypeptide of 567 amino acids. A 5' region (1,028 nucleotides) of STZL cDNA was identical with the ST2 cDNA, and a unique 3' region encoded 'a putative transmembrane domain of 24 amino acids and a cytoplasmic domain of 201 amino acids. The ST2 gene product is highly similar to the extracellular portion of IL-I receptors type 1 and type 2, and the STZL gene product shows a marked similarity with entire IL-l receptor type I.