RNA localization, a regulated step of gene expression, is fundamentally important in development and differentiation. In multidisciplinary experiments, we discovered that RNA (mis)localization underlies the human disease myotonic dystrophy (DM). DM, the most prevalent adult muscular dystrophy, is caused independently by two alleles: DM1 is characterized by a (CTG)n expansion in the DM kinase (DMPK) gene 3' untranslated region while DM2 has a mutation in a small presumptive RNA binding protein. These analyses were guided by disease characteristics and have provided insights to DM's cytopathology, cell biology and molecular genetics. Examining muscle biopsies, it is demonstrated here that DM kinase mRNA is specifically subcellularly localized within normal human muscle and that DM kinase mRNA harboring the 3’UTR mutation (DM1) is mislocalized in DM patient muscle to cytoplasmic areas characteristic of DM disease pathology. Thus, the disease mutation alters the cellular distribution of the effected message. DMPK mRNA mislocalization causes altered DM kinase protein localization, correlates with novel phosphoprotein appearance and can account for DM’s diseased phenotype. While we were fortunate to access DM patient tissue to establish these key findings, the system does not lend itself to experimental manipulation. Hence, I established a disease- relevant tissue culture system, which recapitulates DMPK trafficking, Employing this system; I elucidate a complementary role for the DM2 gene product as a localization factor for DMPK mRNA (DM1 gene product). Comprehensive RNA-protein interaction experiments reveal the DM2 protein specifically and selectively recognizes a small, definitive area within the DMPK RNA 3'UTR. Detailed biochemical, cytological and functional experiments reveal 1) the DM2 protein colocalizes with DMPK mRNA, 2) the small area of the DMPK 3’UTR bound by pDM2 acts to properly localize a reporter construct and 3) disruption of the DM2 protein results in DMPK mRNA mislocalization. These data establish mRNA localization as a vital process underlying human disease etiology. Moreover, they reveal DM1 and DM2 gene products function in the same molecular pathway and that mutation of either causes DMPK mRNA mislocalization, leading to disease. These data have apparent application to several neuromuscular disorders and open a plethora of novel research avenues, both basic and applied. / text