Tetraspanins including CD9, CD37, CD63, and CD151 are linked to cellular adhesion, cell differentiation, migration, carcinogenesis, and tumor progression. The aim of the study was to detect, quantify, and evaluate the prognostic value of these tetraspanins in Merkel cell carcinoma and to study the regulation of CD9 mRNA expression in Merkel cell carcinoma cell lines in detail. Immunohistochemical staining of 28 Merkel cell carcinoma specimens from 25 patients showed a significant correlation of CD9 (P=0.03) and CD151 (P=0.043) expression to overall survival. CD9 and CD63 expression correlated significantly to patients' disease-free interval (P=0.017 and P=0.058). Of primary Merkel cell carcinoma tumors, 42% were CD9 positive in contrast to only 21% of the subcutaneous in-transit metastases. Characterization of the 5' untranslated region (UTR) of the CD9 mRNA from two cultured Merkel cell carcinoma cell lines revealed the presence of two major RNA species differing only in the length of their 5' termini (183 versus 102 nucleotides). In silico analysis of the long CD9 mRNA predicted a 5' UTR folding pattern blocking ribosomal scanning and translation. Quantitative data by real-time RT-PCR not only indicated a reduction of CD9 mRNA but also a distinct quantitative shift toward the long 5' UTR in CD9 receptor negative cells. These observations provide an example for a posttranscriptional fine-tuning of CD9 gene expression in tumor cells.