The mouse H19 gene was identified as an abundant hepatic fetal-specific mRNA under the transcriptional control of a trans-acting locus termed raf. The protein this gene encoded was not apparent from an analysis of its nucleotide sequence, since the mRNA contained multiple translation termination signals in all three reading frames. As a means of assessing which of the 35 small open reading frames might be important to the function of the gene, the human H19 gene was cloned and sequenced. Comparison of the two homologs revealed no conserved open reading frame. Cellular fractionation showed that H19 RNA is cytoplasmic but not associated with the translational machinery. Instead, it is located in a particle with a sedimentation coefficient of approximately 28S. Despite the fact that it is transcribed by RNA polymerase II and is spliced and polyadenylated, we suggest that the H19 RNA is not a classical mRNA. Instead, the product of this unusual gene may be an RNA molecule.
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