Background: Tenascin-C (Tn-C) is an extracellular matrix protein with multiple functions that is present at low levels in normal tissues, but which is highly present in various tumours. The mRNA expression and protein level of Tn-C including its various isoforms have not been investigated comprehensively so far in cutaneous squamous cell carcinoma (SCC) and the precursor lesion actinic keratosis (AK). Objectives: To assess the dysregulated expression and splice variants of Tn-C in cutaneous squamous cell dysplasia and carcinoma. Methods: Biopsies from 66 patients (or representative subsets) that comprised 25 specimens from normal skin, 19AK and 22 cutaneous SCC were analysed for Tn-C splice variants using splice-specific primers. The amount of Tn-C mRNA was investigated by quantitative real-time reverse transcription-polymerase chain reaction. In addition, the presence of Tn-C protein was analysed in sections of paraffin-embedded tissues using immunohistochemistry. Results: The large Tn-C splice variant was present in only 5%of normal skin samples, in comparison with 63%of AK (P < 0.001) and 88%of SCC (P < 0.001). Tn-C mRNA expression was significantly increased in AK and SCC compared with normal skin (P < 0.001). The corresponding proteins were rarely detected in cells of the vascular epithelial layers and perifollicular layers of some normal skin specimens, and their spatial localization expanded into the papillary dermis of AK. The largest amount and the widest distribution were found in samples of SCC, in which Tn-C was located in the basal cells at the tumour invasion front and additionally in the papillary dermis and reticular dermis. Conclusions: Tn-C is present in the dermis, its expression is increased during skin cancer development, and the large splice variant is characteristic for AK and SCC, which may prove useful for diagnostic approaches in cutaneous SCC. Background: Tenascin-C (Tn-C) is an extracellular matrix protein with multiple functions that is present at low levels in normal tissues, but which is highly present in various tours. The mRNA expression and protein level of Tn-C including its various isoforms have not been investigated comprehensively so far in cutaneous squamous cell carcinoma (SCC) and the precursor lesion actinic keratosis (AK). Objectives: To assess the dysregulated expression and splice variants of Tn-C in cutaneous squamous cell dysplasia and carcinoma. Methods: Biopsies from 66 patients (or representative subsets) that were 25 specimens from normal skin, 19AK and 22 cutaneous SCCs were analyzed for Tn-C splice variants using splice-specific primers. The amount of Tn-C mRNA was investigated by quantitative real-time reverse transcription-polymerase chain reaction. In addition, the presence of Tn-C protein was analyzed in sections of paraffin-embedded tissues using immunohistochemistry. Results: The large T nC splice variant was present in only 5% of normal skin samples, in comparison with 63% of AK (P <0.001) and 88% of SCC (P <0.001). Tn-C mRNA expression was significantly increased in AK and SCC with normal skin (P <0.001). The corresponding proteins were rarely detected in cells of the vascular epithelial layers and perifollicular layers of some normal skin specimens, and their spatial localization expanded into the papillary dermis of AK. The largest amount and the widest distribution were found in samples of SCC, in which Tn-C was located in the basal cells at the invasion of front and further in the papillary dermis and reticular dermis. Conclusions: Tn-C is present in the dermis, its expression is increased during skin cancer development, and the large splice variant is characteristic for AK and SCC, which may prove useful for diagnostic approaches in cutaneous SCC.