This document discusses a study that found constitutive membrane association greatly enhances the ability of Bruton's tyrosine kinase (Btk) to transform cells. The study targeted Btk to the plasma membrane using fusion proteins, which led to high tyrosine phosphorylation of Btk. Cell transformation required membrane localization of Btk, its kinase activity, phosphorylation by Src kinases, and an intact SH2 domain. The results suggest membrane localization is a critical early step in Btk activation.