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Objective: About 20% of sporadic medullary thyroid carcinomas (MTC) have no RET/RAS somatic alterations or other known gene alterations. The aim of this study was to investigate RET/RAS-negative MTC for the presence of NF1 alterations. Methods: We studied 18 sporadic RET/RAS-negative MTC cases. Next-generation sequencing (NGS) of tumoral and blood DNA was performed using a custom panel including the entire coding region of the NF1 gene. The effect of NF1 alterations on the transcripts was characterized by reverse transcriptase-polymerase chain reaction (RT-PCR), and the loss of heterozygosity (LOH) of the other NF1 allele was investigated with Multiplex Ligation-dependent Probe Amplification (MLPA). Results: Two cases showed biallelic inactivation of NF1 with a prevalence of about 11% of RET/RAS-negative cases. In a patient affected by neurofibromatosis, there was a somatic intronic point mutation determining the transcript alteration in 1 allele and a germline LOH in the other. In a second patient, we described that both the point mutation and the LOH were somatic events; this latter finding shows, for the first time, a driver role of NF1 inactivation in MTC independent of RET/RAS alterations and the presence of neurofibromatosis. Conclusions: About 11% of our series of sporadic RET/RAS-negative MTC harbor biallelic inactivation of the NF1 suppressor gene also regardless of neurofibromatosis status. According to our results, NF1 alterations should be searched in all RET/RAS-negative MTC as possible drivers. Moreover, this finding reduces the number of negative sporadic MTC and may have important clinical implications in the management of these tumors. [ABSTRACT FROM AUTHOR]