IBs form rapidly in a concentration- and repeat length–dependent manner. (A) U2OS cells were transiently transfected with N-htt(Q91)–chFP, and formation of IBs was followed by time-lapse fluorescence video microscopy at 6-min intervals. Note the absence of diffuse mCherry fluorescence outside of the IB at the 30-min time point (Video 1). The arrowheads indicate the site of a forming IB. Bars, 25 µm. (B) Threshold for IB formation. Cells transiently transfected with N-htt(Q91)–chFP or N-htt(Q47)–chFP were monitored by time-lapse fluorescence video microscopy as in A. Total chFP fluorescence intensity in individual cells in the frame just before the first detectable IB was quantified and plotted. The data shown are from a single representative experiment out of two independent repeats.