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Transformation of a source schema to a target schema is an important activity in data integration. In XML data transformation for data integration purposes, when an XML source schema with its conforming data is transformed to the target XML schema, one of the important XML constraints, XML keys that are defined on the source schema for expressive semantics can also be transformed. Thus, whether keys should be transformed and preserved, and if not preserved, whether keys can be captured in another form of XML constraints are important questions. To answer these questions, first, we define XML keys and XML functional dependencies(XFD) on document type definition (DTD). Second, we show key preservation in transformation. If keys are not preserved, we then show how to capture them as XFDs. We term this as key transition. Our research about the XML key preservation and transition is towards handling the issues of integrity constraints in XML data integration.