〔Abstract〕 Objective To explore the role and its clinical significance of P300 in biphasic differentiation and epithelial mesenchymal transition(EMT) process in synovial sarcoma(SS) by detecting the expression of histone acetyltransferase P300 and EMT related molecules. Methods 40 cases of SS paraffin embedded tissue samples were collected, and the SYT-SSX fusion gene subtype was detected by RT-PCR. Immunohistochemistry was used to detect the expression of P300 and EMT-related molecules. Results (1)The positivity of SYT-SSX fusion gene was 92.5%(37/40), including 90%(27/30) of BSS(66.7% SYT-SSX1, 33.3% SYT-SSX2) and 100%(10/10) of MFSS(SYT-SSX1 80%, SYT-SSX2 20%), there was no correlation between histological subtype and fusion gene subtype in SS.(2)The rate of positive expression of P300 was 95%(38/40), including 93.3%(28/30) of BSS and 100%(10/10) of MFSS, and there was no significance difference between the expression P300 and histological subtype.(3)The expression of P300 was not statistically correlated with clinicopathological parameters of SS, however, COX regression analysis showed that tumour metastasis(P=0.019) and TNM stage(Ⅲ～Ⅳ)(P=0.003) were independent risk factors for overall survival of patient.(4)The expression of P300 was significantly related with EMT-associated protein β-catenin in the BSS(P=0.027) and Slug in the MFSS(P=0.048), which suggested P300 was closely related to EMT. Conclusion P300 may be involved in the biphasic differentiation and the EMT process of SS, suggesting that P300 plays a certain role in SS invasion and migration.