on metadata and other specifications. In July 2009 the CellML website was completely revamped, and an initial version of the new CellML repository software (PMR2) was released. The structure of a CellML model A CellML model consists of a number of components, each described in their own component element. A component can be an entirely conceptual entity created for modelling convenience, or it can have some real physical interpretation (for example, it could represent the cell membrane). Each component contains a number of variables, which must be declared by placing a variable element inside the component. For example, a component representing a cell membrane may have a variable called V representing the potential difference (voltage) across the cell membrane. Mathematical relationships between variables are expressed within components, using MathML. MathML is used to make declarative expressions (as opposed to procedural statements as in a computer programming language). However, most CellML processing software will only accept a limited of range of mathematics (for example, some processing software requires equations with a single variable on one side of an equality). The choice of MathML makes CellML particularly suited for describing models containing differential equations. There is no mechanism for the expression of stochastic models or any other form of randomness. Components can be connected in other components using a connection element, which describes the name of two components to be connected, and the variables in the first component which are mapped to variables in the second component. Such connections are a statement that the variable in one component is equivalent to another variable in another component. CellML models also allow relationships between components to be expressed. The CellML specification defines two types of relationship, encapsulation and containment, however more can be defined by the user. The containment relationship is used to express that one component is physically within another. The encapsulation relationship is special because it is the only relationship that affects the interpretation of the rest of the model. The effect of encapsulation is that components encapsulated beneath other components are private and cannot be accessed except by the component directly above in the encapsulation tree. The modeller is free to use encapsulation as a conceptual tool, and it does not necessarily have any physical interpretation. Specifications CellML is defined by core specifications as well as additional specifications for metadata, used to annotate models and specify simulations. CellML 1.0 CellML 1.0