Objective To cast light on the mechanisms governing anaplastic thyroid carcinoma (ATC) and to evaluate the minichromosomes maintain protein (MCM) 5 and MCM7 expression in human normal thyroid (NT),papillary thyroid carcinoma (PTC),and ATC samples,as well as in primary culture cells.Methods We tested the expression of MCM5,MCM7and PCNA in NT,PTC and ATC by immunohistochemistry.We used Western blot and Northern blot to test the expression of MCM5,and MCM7.ATC cells were transfected with MCM7 siRNA,and Western blot was used to examine the change of the expression of MCM7.Results In ATC samples,MCM5 had high expression in 65％ patients,and MCM7 had high expression in 73％ patients.The expression of MCM5 and MCM7 in NT and PTC samples were very low which can be ignored.In ATC cells,high MCM5 and MCM7 expression was paralleled by high levels of MCM2 and MCM6.Inhibition of MCM7 protein levels by small inhibitory duplex RNAs could reduce the rate of DNA synthesis in ATC cells.Conclusion MCM protein expression is upregulated in ATC and leads to excessive proliferation of ATC.