library_construction_protocol: CUT&Tag was performed using the Hyperactive Universal CUT&Tag Assay Kit for Illumina (Vazyme) following the manufacturer's protocol. Briefly, Cardiomyocytes were isolated from mouse hearts at postnatal days 1 (P1) and 8 (P8),  and harvested after PBS wash. Cardiomyocytes  (1 × 10⁵) were bound with concanavalin A-coated  magnetic beads and incubated with a primary antibody against RBM22 (Bethyl) at 4 °C overnight. On the next day, samples were incubated with a second antibody and then with Hyperactive pA/G-Transposon for 1 h. DNA was treated with TTBL at 37 °C for 1 h and extracted. The librarys were constructed by purified DNA, and amplified using Hyperactive Universal CUT&Tag Assay Kit for Illumina (Vazyme) according to the product instructions.