Validation of a CD81-based flow cytometry assay to assess dCas9 silencing activity

The efficiency of CRISPR interference (CRISPRi) depends on functional dCas9 activity, yet practical and reproducible validation of dCas9-expressing cell lines remains limited. Here, we describe a simple and reproducible assay to assess dCas9 functionality using a single sgRNA targeting the nonessential and ubiquitously expressed surface protein CD81. We evaluated this approach in multiple hematological and solid tumor cell lines expressing the dCas9-KRAB-MeCP2 repressor complex. In all tested models, CD81 targeting resulted in a consistent reduction of surface protein levels, quantified by flow cytometry. This assay provides a rapid and quantitative functional readout of dCas9 activity without the need for reporter constructs or transcriptional assays. The CD81-targeting sgRNA and validated cell lines are made available to support reproducibility and technical standardization in CRISPRi experiments. This strategy can be readily implemented in any laboratory using CRISPRi-based approaches.