A third member of the thrombospondin gene family (Thbs3) has been partially characterized in the mouse. In both the mouse and humans, the Thbs3/THBS3 gene is located immediately upstream from the Muc1/MUC1 (episialin) gene; less than 3 kilobases separate the polyadenylation signal of one gene from the start of transcription of the other. The available coding sequence in Thbs3 shows a high degree of amino acid sequence identity to Thbs1 and Thbs2 (58 and 59%, respectively, in exons 15 and 16), but the exon/intron organization of Thbs3 appears to be more disparate than that of the two previously described members of the family. The shorter length of the Thbs3 mRNA (3.5 kilobases) can be attributed largely to a shorter 3'-untranslated region. The Thbs3 gene is expressed in a distinctive pattern in mouse tissues, with the highest level of expression in lung. This pattern suggests a unique function for the translation product of the Thbs3 gene.