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Objective:To investigate the clinical significance of MAPT-IT1 in breast cancer and its biological effect in vitro.Methods:The expression of MAPT-IT1 in breast cancer was analyzed by TCGA database. 64 cases of breast cancer and normal adjacent tissues were collected. Human breast cancer MDA-MB-231 and MCF-7 cells were cultured and overexpressed MAPT-IT1 or rescue miR-181a-5p by cell transfection. MDA-MB-231 cells were divided into blank group A, overexpression group A and recovery group A; MCF-7 cells were divided into blank group B, overexpression group B and recovery group B. Real time quantitative PCR (RT-qPCR) was used to detect the expression of MAPT-IT1, miR-181a-5p and MAPT mRNA. Western blot was used to detect the expression of MAPT protein. CCK-8 assay was used to detect cell proliferation, and Transwell invasion assay was used to detect cell invasion.Results:The expression of MAPT-IT1 in normal paracancerous tissues and breast cancer tissues was 0.011±0.002 and 0.028±0.003 respectively. The expressions of MAPT-IT1 in breast cancer tissues were significantly higher than those in adjacent normal tissues, and high expression of MAPT-IT1 was correlated with early tumor progression, ER positive and prolonged prognosis ( P<0.05) . In blank group A, overexpression group A and recovery group A, the expressions of MAPT-IT1 were 1.000±0.078, 8.597±0.320 and 8.540±0.177, miR-181a-5p were 1.000±0.027, 0.263±0.024, 4.433±0.239, MAPT were 1.000±0.071, 3.297±0.243, 0.497±0.029. In blank group B, overexpression group B and recovery group B, the expressions of MAPT-IT1 were 1.000± 0.081, 5.716±0.309, 5.288±0.176, miR-181a-5p were 1.000±0.024, 0.291±0.022, 3.648±0.073, and MAPT were 1.000±0.054, 3.309±0.177, 0.883±0.075. After overexpression of MAPT-IT1, the expression of miR-181a-5p was down-regulated, while the expression of MAPT was significantly increased ( P<0.001) , and the proliferation and invasion ability of MDA-MB-231 and MCF-7 cells were significantly decreased ( P<0.05) . Re-expression of miR-181a-5p down-regulated MAPT and promoted cell proliferation and invasion ( P<0.05) . Conclusion:Overexpression of MAPT-IT1 can significantly down-regulate the expression of miR-181a-5p and enhance MAPT and inhibit the malignant phenotype of breast cancer cells in vitro.