T-circle detection by 2-dimensional neutral-neutral gels and T-circle amplification assays. (A) Two-dimensional gel analysis of telomeric DNA from a T-circle positive control (SV40 transformed fibroblasts overexpressing the telomerase TERT-encoding gene) reveals an additional signal corresponding to T-circles. Two-dimensional gel analysis of telomeric DNA from SV40-transformed fibroblasts from P1 and activated T cells (T-blasts) from P3 did not reveal T-circle signal. (B) Detection of T-circles by T-circle amplification in different positive controls (ie, alternative lengthening of telomere–positive cells SUSM1, VA13, and U2OS), but not in SV40-fibroblats, T-blasts, B lymphoblastoid cell line, and blood cells from healthy controls and RTEL1-deficient patients P1-P8.