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OBJECTIVE To investigate the expression of long non-coding RNA (lncRNA) DARS-AS1 in thyroid cancer, and to further investigate whether it can promote the development of thyroid cancer by regulating microRNA-129. PATIENTS AND METHODS Real-time quantitative polymerase chain reaction (qPCR) was used to detect the level of DARS-AS1 in tumor tissues and paracancerous tissues of 34 thyroid carcinomas. It was also used to analyze the relationship between the expression of DARS-AS1 and the clinical indicators of thyroid cancer and the prognosis of patients. qPCR was used to further verify the expression of DARS-AS1 in thyroid cancer cell lines. The DARS-AS1 knockdown model was constructed using lentivirus in thyroid cancer cell lines. Cell counting kit-8 (CCK-8), cell clone formation, and transwell migration assays were performed to evaluate the effects of DARS-AS1 on the biological function of thyroid cancer cells. Finally, the potential mechanism was explored by using recovery experiments and the interplay between DARS-AS1 and microRNA-129 was further studied. RESULTS qPCR results revealed that the level of DARS-AS1 in tumor tissues of thyroid cancer patients was remarkably higher than that in adjacent tissues, and the difference was statistically significant. Compared with patients with low expression of DARS-AS1, patients with high DARS-AS1 expression had a higher incidence of high tumor stage, distant metastasis, and a lower overall survival rate. Besides, compared with NC group, the proliferation and migration ability of shRNA-AS1 expression knockdown group sh-DARS-AS1 was remarkably decreased. qPCR results indicated that there was a negative correlation between the level of microRNA-129 and DARS-AS1 in thyroid cancer tissues. In addition, cell proliferation and migration ability in the microRNA-129 overexpression group were remarkably decreased. The recovery experiment also found that there was a mutual regulation between DARS-AS1 and microRNA-129, which together affected the malignant progression of thyroid cancer. CONCLUSIONS DARS-AS1 level in tumor tissues of thyroid cancer was remarkably increased and was correlated with the pathological stage, distant metastasis, and poor prognosis of thyroid cancer. Moreover, DARS-AS1 could promote the proliferation and migration capabilities of thyroid cancer cells by modulating microRNA-129.