Principle of the base editor activity reporter (BEAR) assay. BEAR consists of a split GFP coding sequence (green) separated by an intron of which splice donor site is altered to ‘AC’, resulting in a dysfunctional protein (grey). This inactive splice donor site can be rescued either by ABEs reverting the ‘AC’ splice donor site to ‘AT’ or by CBEs reverting the ‘AC’ splice donor site to ‘GC’, respectively. ‘AT’ and ‘GC’ are known to be functional non-canonical splice donor sites in the human genome.