CD177 and myeloid leukemia: To evaluate the performance and feasibility of CRISPRa-mediated transcriptional activation of cell surface proteins for a receptor/ligand interaction discovery platform, we first selected 10 well-characterized cell surface receptors with varying mRNA expression levels ranging from not detected to highly expressed in K562 human myeloid leukemia cells (Figure 1—figure supplement 1A; Thul et al., 2017; Uhlén et al., 2019).