Protein coding structural genes in higher eukaryotes are transcribed in the nucleus, but the RNA transcript in the nucleus differs from mRNA used for translation in the cytoplasm. This RNA transcript in the nucleus, due to its broad size distribution (500-20,000 bp; mean length 2000 bp), is called heterogeneous nuclear RNA (hnRNA) and is much larger than its corresponding mRNA. The fate of this hnRNA can be orfe of the following : (i) RNA transcripts of some genes do not seem to give rise to any cytoplasmic mRNA, but get degraded within the cytoplasm, (ii) For each gene, only a small proportion (25%) of RNA transcripts takes part in RNA processing leading to formation of mRNA, the remaining 75% undergoing degradation in the nucleus. Due to these two events only 5% of hnRNA (by mass) enters the cytoplasm. This also suggests that regulation of gene expression is also exercised at the level of hnRNA. The hnRNA molecules, which are destined to produce mRNA, undergo RNA processing which includes the following events : (i) Modification of 5' end by capping (addition of 7-metal guanine) and modification of 3' end by a tail (polyadenylation) after enzymatic cleavage; (ii) Splicing out of intron sequences from RNA transcripts of interrupted genes. It has been shown that cleavage and polyadenylation usually precede RNA splicing, although there is no causal relationship between the two.