HnRNPs L, E2, and PTB are identified as ESS1-associated proteins by RNA affinity purification. (A) Affinity purification of proteins associated with ESS1 RNA. Silver stained SDS–PAGE gel of proteins isolated from JSL1 nuclear extract by virtue of association with chemically synthesized, 5′-biotinylated ESS1, mESS, and E14 RNAs (see Materials and methods for details). (B) Western blot of SDS–PAGE gel from panel A with specified antibodies. (C) Sequence of ESS1 with predicted high-affinity binding sites indicated for hnRNP L (bold lines), PTB (dotted lines), and hnRNP E (plain lines).