CONTEXT: Micro-RNA have emerged as an important class of short endogenous RNA that act as posttranscriptional regulators of gene expression and are constantly deregulated in human cancer. MiR-1 has been found down-regulated in lung, colon, and prostate cancer. OBJECTIVES: In this study, we investigated the possible role of miR-1 in thyroid carcinogenesis. DESIGN: We have analyzed miR-1 expression in a panel of thyroid neoplasias including benign and malignant lesions and searched for miR-1 targets. RESULTS: Our results show that miR-1 expression is drastically down-regulated in thyroid adenomas and carcinomas in comparison with normal thyroid tissue. Interestingly, miR-1 down-regulation was also found in thyroid hyperproliferative nonneoplastic lesions such as goiters. We identified the CCND2, coding for the cyclin D2 (CCND2) protein that favors the G1/S transition, CXCR4, and SDF-1α genes, coding for the receptor for the stromal cell derived factor-1 (SDF-1)/CXCL12 chemokine and its ligand SDF-1/CXCL12, respectively, as miR-1 targets. An inverse correlation was found between miR-1 expression and CXC chemokine receptor 4 (CXCR4) and SDF-1α protein levels in papillary and anaplastic thyroid carcinomas. Consistent with a role of the CCND2 protein in cell proliferation and CXCR4 and SDF-1α proteins in cell invasion and metastasis, functional studies demonstrate that miR-1 is able to inhibit thyroid carcinoma cell proliferation and migration. CONCLUSIONS: These results indicate the involvement of miR-1 in thyroid cell proliferation and migration, validating a role of miR-1 down-regulation in thyroid carcinogenesis.