Term: pro-E, pro-Z https://doi.org/10.1351/goldbook.P04867 Definition: One of a pair of identical groups c attached to a double bond (as in abC=Cc2) is described as pro-E if, when it is arbitrarily assigned CIP priority over the other group c, the stereodescriptor of the molecule becomes E. The other group c is then described as pro-Z. Related Terms: 1) cip priority (http://doi.org/10.1351/goldbook.C01082). 2) stereodescriptor (http://doi.org/10.1351/goldbook.S05976). Source: PAC, 1996, 68, 2193. 'Basic terminology of stereochemistry (IUPAC Recommendations 1996)' on page 2214 (https://doi.org/10.1351/pac199668122193) Citation: 'pro-E, pro-Z' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.P04867 License: The IUPAC Gold Book is licensed under Creative Commons Attribution-ShareAlike CC BY-SA 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) for individual terms. Disclaimer: The International Union of Pure and Applied Chemistry (IUPAC) is continuously reviewing and, where needed, updating terms in the Compendium of Chemical Terminology (the IUPAC Gold Book). Users of these terms are encouraged to include the version of a term with its use and to check regularly for updates to term definitions that you are using.