Dear PDB-l We are excited to announce the launch of a detailed PDBx/mmCIF format user guide <https://mmcif.wwpdb.org/docs/user-guide/guide.html>. As the foundation for depositing, annotating, and archiving structural data across diverse experimental techniques, the Protein Data Bank Exchange macromolecular Crystallographic Information Framework (PDBx/mmCIF) stands as a master format of the Protein Data Bank. Our user-friendly guide offers detailed explanation and examples of essential PDBx/mmCIF records, aimed to facilitate a smooth transition to this format for depositors and users alike. The wwPDB anticipates that all four-character PDB IDs will be exhausted by 2028, after which 12-character PDB IDs will be issued. Entries with extended PDB IDs will not be compatible with the legacy PDB file format and will only be available in PDBx/mmCIF format. wwPDB encourages users to transition to the PDBx/mmCIF format as soon as possible. We invite all users to participate in a brief survey <https://rutgers.ca1.qualtrics.com/jfe/form/SV_ebusbrMCmLbB0WO> which is accessible at PDBx/mmCIF format user guide <https://mmcif.wwpdb.org/docs/user-guide/guide.html> to share their feedback on this PDBx/mmCIF user guide by December 15, 2024. Your feedback will greatly contribute to future developments. -- YUHE LIANG, Ph.D. Biocuration Lead Deputy, RCSB Protein Data Bank Research Associate, Institute for Quantitative Biomedicine Rutgers, The State University of New Jersey 174 Frelinghuysen Road, Piscataway NJ 08554 P: 848.445.4938 | E: yuhe.liang@rcsb.org