The assay for transposase-accessible chromatin using sequencing (ATAC-seq) is 
widely used to identify regulatory regions throughout the genome. However, very 
few studies have been performed at the single cell level (scATAC-seq) due to 
technical challenges. Here we developed a simple and robust plate-based 
scATAC-seq method, combining upfront bulk Tn5 tagging with single-nuclei 
sorting. We demonstrate that our method works robustly across various systems, 
including fresh and cryopreserved cells from primary tissues. By profiling over 
3000 splenocytes, we identify distinct immune cell types and reveal cell 
type-specific regulatory regions and related transcription factors.