Abstract¡G Activating point mutations of the BRAF gene have been recently reported to be restricted to papillary thyroid carcinomas (PTCs) and anaplastic thyroid carcinomas (ATCs) arising from PTCs among various benign and malignant thyroid tumors. A thymine-to-adenine transversion at nucleotide 1799 (T1799A), formerly designated as T1796A, in exon 15 resulting in a valine to glutamate substitution at residue 600 (V600E), formerly designated as V599E, was the only mutational site reported in thyroid cancer. We have previously shown that BRAFV600E mutations were detected in 49 of 105 (47%) PTCs but not in 5 follicular thyroid carcinomas (FTCs), 10 follicular adenomas, 3 Hürthle cell adenomas and 10 nodular goiters. In contrast to PTC, to date only few studies have been published concerning the frequency of BRAFV600E in ATCs and their coexisting differentiated thyroid carcinomas. In addition, there is no report concerning the BRAF status in multiple PTCs. In this study, paraffin-embedded tumor tissues of 25 patients with thyroid cancer (15 ATCs and 10 PTCs) were obtained from the Department of Pathology of Chang Gung Memorial Hospital-Kaohsiung. Nine ATCs were found to contain a coexisting differentiated carcinoma, including three follicular and six papillary carcinomas. Ten cases of PTC with multiple tumor foci were selected from a cohort of 105 PTCs as previously reported. Paraffin blocks containing tumor were sectioned followed by microdissection to obtain tissue for DNA extraction. Mutational hot spot in exon 15 (codon 600) of the BRAF gene were amplified by PCR and sequenced with an automatic sequencer. BRAFV600E mutations were detected in 3 of 15 (20%) ATCs. In the 9 coexisting differentiated carcinomas, 2 out of 6 PTCs harbored BRAF mutations but not in the three follicular carcinomas. In none of 3 ATCs with coexisting PTCs were mutations detectable in both tumor types. Among the ten cases of PTCs with multiple tumor foci, 5 cases demonstrated the same BRAF status in each tumor foci whereas 5 cases showed distinct BRAF status in different tumor foci. We conclude that the distinct BRAF status in anaplastic carcinoma and its coexisting differentiated carcinoma suggests that anaplastic carcinoma might not arise from differentiated carcinoma.The distinct BRAF status in different tumor foci of multiple papillary carcinomas suggests that multifocal tumors might not be formed through intrathyroidal lymphatic metastasis.