In this study we present two novel anaplastic thyroid carcinoma (ATC) lines (HTh 104 and HTh 112) and further characterize six frequently used ATC lines (HTh 7, HTh 74, HTh 83, C 643, KAT-4, and SW 1736). Three of the lines carried a heterozygous BRAF mutation V600E, which is in line with reports of BRAF mutations in primary ATC and papillary thyroid cancer. Several nonrandom breakpoints were identified by spectral karyotyping (SKY) and G-banding in these lines including the novel 1p36 and 17q24-25 as well as 3p21-22 and 15q26 that are also implicated in well-differentiated thyroid cancers. Comparative genomic hybridization showed frequent gain of 20q, including the UBCH10 gene in 20q13.12, which was further confirmed by array-comparative genomic hybridization and fluorescence in situ hybridization analyses. Our results concur with previous studies in both primary tumors and cell lines, indicating that gain of chromosome 20 is important in the pathogenesis of ATC and/or progression of differentiated thyroid cancers to ATC.