Abstract
Transcription is inherently stochastic even in clonal cell populations 1. Studies at single-cell-single-molecule level enable a quantitative understanding of the underlying regulatory mechanisms 2,3. A widely used technique is single-molecule RNA fluorescence in-situ hybridization (FISH), in which fluorescent probes target the mRNA of interest and individual molecules appear as bright diffraction-limited spots (Fig. 1a,b) 3. Recent experimental progress makes FISH easy to use 4 , but a dedicated image analysis tool is currently missing. Available methods allow counting of isolated mature mRNAs but cannot reliably quantify the dense mRNA aggregates at transcription sites (TS) in three dimensions (3D), particularly of highly transcribing genes 4. We developed FISH-QUANT to close this gap (Supplementary Note 1)