Proteins that directly bind to nascent RNA polymerase II transcripts, the heterogenous nuclear ribonucleoproteins (hnRNPs), play an important role in both transcript-specific packaging and alternative splicing of pre-mRNAs. A group of abundant hnRNPs, the M1-M4 proteins, appear as a cluster of four proteins. The M proteins are pre-mRNA binding proteins in vivo, and they bind avidly to poly (G) and poly (U) RNA homopolymers in vitro. M proteins are members of the ribonucleoprotein consensus sequence family of RNA-binding proteins with greatest similarity to a hypothetical RNA-binding protein from Saccharomyces cerevisiae. The M proteins also possess an unusual hexapeptide-repeat region rich in methionine and arginine residues (MR repeat motif) that resembles a repeat in the 64kD subunit of cleavage stimulation factor, which is involved in 3-end maturation of pre-mRNAs. Proteins immunologically related to M exist in divergent eukaryotes ranging from human to yeast.