DNMT1 and cancer: 5‐Methylcytosine (5mC, Figure 1 a) is a dynamic regulatory element of mammalian genomes with key roles in transcription regulation, differentiation and development.[1] 5mC is introduced and maintained by DNA‐methyltransferases (DNMT), and DNMT mutations are an early event in carcinogenesis.[2] The ability to control DNMTs with spatiotemporal resolution in vivo would enable precise kinetic insights into how cancer‐related mutations alter DNMT function.