(A) Yeast two-hybrid assay. Positive interaction between the RAG1 N-terminal domain and any member of the library of fusion proteins allows growth under histidine-restrictive conditions (plates 1–3). In contrast, all cells grow on control double drop-out plates (plate 4). (B) Three proteins were isolated, including the previously known Karyopherin α2. (C) The isolated fragment of SF3A2 is shown. It is composed largely of imperfect repeats of a proline-rich heptad PPAPGVH.