This document discusses the Expression Transformer component in Mule. It can evaluate one or more expressions on the current message and set the result as the new payload. Key points include: - It requires an expression evaluator and expression to run. Expressions can return null or multiple values. - The Advanced tab allows configuring return arguments with custom evaluators and expressions. - An example flow is described that uses expressions to set properties, pass data between flows, transform payloads, and route based on expression results.