Heterogeneous nuclear ribonucleoproteins (hnRNP) bind to nascent RNA polymerase II transcripts and are involved in transcript-specific packaging as well as alternative splicing of pre-mRNAs. hnRNPs also appear to influence mRNA metabolism and transport. In vivo, the M proteins bind to mRNA, and in vitro they bind avidly to poly(G) and poly(U) homopolymers. Four different isoforms of hnRNP are produced by alternative splicing: M1, M2, M3, and M4. Ongoing studies have theorized that hnRNPs may cause certain genetic diseases such as myotonic dystrophy, which is the most common form of adult muscular dystrophy. The identification of hnRNPs may therefore be important to determining the cause and cure for genetic diseases.