VRS describes several classes for representing biological sequence variation. Classes are shown as boxes. Inheritance and composition are shown with lines connecting classes. Dashed borders denote abstract classes, which are not instantiated. Four abstract classes – Variation, Location (Abstract Class), State (Abstract Class), and Interval (Abstract Class) – enable specializations of concepts in this and future versions of VRS. Bold borders denote identifiable classes – that may be referenced with an identifier. Identifiable objects have an optional _id attribute. Thin solid borders denote classes that not identifiable; these classes exist only to structure data within identifiable classes. All classes have a string type. Dashed arrow lines denote inheritance; subclasses inherit all attributes from their parent. Inherited attributes are not shown in this diagram. Solid lines with diamonds denote composition of one class with objects from another. An asterisk on a class attribute definition denotes an attribute that may contain either the object or a CURIE identifier to that object.