P.d.b. Apr 2026

The is the worldwide repository for the 3D structural data of large biological molecules, such as proteins and nucleic acids. This guide provides the essentials for navigating and understanding its data. 1. Core Concepts & Identifiers

: An "Entity" is a unique molecule (like a specific protein chain), while an "Instance" (or Chain ID) is a specific copy of that molecule within the structure (e.g., Chain A and Chain B). 2. Standard File Formats P.D.B.

: The functional form of a molecule (e.g., a four-unit hemoglobin complex), which may differ from the Asymmetric Unit (the smallest portion used to solve the crystal structure). The is the worldwide repository for the 3D

The archive uses several formats to store 3D coordinates and experimental data: Guide to Understanding PDB Data: Biological Assemblies Core Concepts & Identifiers : An "Entity" is

: Every entry is assigned a unique four-letter/number code (e.g., 4HHB for hemoglobin).