Objective To investigate the expression of common-partner Smad—Smad4 in psoriatic lesions.   Methods The skin of psoriatic lesion from 21 patients visiting our out-patient clinic or admitted in Department of Dermatology of our hospital (13 males and 8 females

aged 10-67 years old

averaging 33.7)  and the normal skin from 20 healthy subjects who underwent plastic surgery or bone surgery (13 males and 7 females

aged 13-59 years old

averaging 29.3). Quantitative real-time PCR and immunohistochemical technique were used to detect the expression levels of Smad4.   Results The protein and mRNA expression levels of Smad4 were significantly lower in psoriatic lesions than those in normal control skin.   Conclusion Down-regulated expressions of Smad4 in psoriatic epidermis might contribute to the formation of hyperplastic epidermis in psoriatic lesions through blocking TGF-β signal transduction.