In addition, CK1 kinase inhibition with D4476 resulted in the accumulation of EWS–FLI1 in multiple Ewing sarcoma cells (Figure 2B–D and Figure S4A,B (Supporting Information)) without significantly affecting EWS–FLI1 mRNA levels (Figure S4C–E, Supporting Information). Here, FLI1 is linked to Ewing sarcoma.