Journal of Integrative Bioinformatics 2023; 20(1): 20230003 Michael Clerx, Michael T. Cooling, Jonathan Cooper, Alan Garny, Keri Moyle, David P. Nickerson*, Poul M. F. Nielsen and Hugh Sorby CellML 2.0.1 https://doi.org/10.1515/jib-2023-0003 Received January 31, 2023; accepted February 28, 2023; published online March 17, 2023 Abstract: We present here CellML 2.0.1, an XML-based language for describing and exchanging mathematical models of physiological systems. MathML embedded in CellML documents is used to deﬁne the underlying math- ematics of models. Models consist of a network of reusable components, each with variables and equations giving relationships between those variables. Models may import other models to create systems of increasing complexity. CellML 2.0.1 is deﬁned by the normative speciﬁcation presented here, prescribing the CellML syntax and the rules by which it should be used. The normative speciﬁcation is intended primarily for the developers of software tools which directly consume CellML syntax. Users of CellML models may prefer to browse the infor- mative rendering of the speciﬁcation (https://cellml.org/speciﬁcations/cellml_2.0/) which extends the normative speciﬁcation with explanations of the rules combined with examples of their usage. This version improves the identiﬁcation of rule statements and corrects errata present in the CellML 2.0 speciﬁcation. *Corresponding author: David P. Nickerson, University of Auckland, Auckland, New Zealand, E-mail: d.nickerson@auckland.ac.nz. https://orcid.org/0000-0003-4667-9779 Michael Clerx, University of Nottingham, Nottingham, UK, E-mail:michael.clerx@nottingham.ac.uk Michael T. Cooling, Alan Garny, Keri Moyle, Poul M. F. Nielsen and Hugh Sorby, University of Auckland, Auckland, New Zealand, E-mail: m.cooling@auckland.ac.nz (M. T. Cooling), a.garny@auckland.ac.nz (A. Garny), keri.moyle@gmail.com (K. Moyle), p.nielsen@auckland.ac.nz (P. M. F. Nielsen), h.sorby@auckland.ac.nz (H. Sorby) Jonathan Cooper, University College London, London, UK, E-mail: j.p.cooper@ucl.ac.uk (J. Cooper) Open Access. © 2023 the author(s), published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 International License.