Intronless genes (IGs) constitute approximately 3% of the human genome. Human 
IGs are essentially different in evolution and functionality from the IGs of 
unicellular eukaryotes, which represent the majority in their genomes. 
Functional analysis of IGs has revealed a massive over-representation of signal 
transduction genes and genes encoding regulatory proteins important for growth, 
proliferation, and development. IGs also often display tissue-specific 
expression, usually in the nervous system and testis. These characteristics 
translate into IG-associated diseases, mainly neuropathies, developmental 
disorders, and cancer. IGs represent recent additions to the genome, created 
mostly by retroposition of processed mRNAs with retained functionality. 
Processing, nuclear export, and translation of these mRNAs should be hampered 
dramatically by the lack of splice factors, which normally tightly cover mature 
transcripts and govern their fate. However, natural IGs manage to maintain 
satisfactory expression levels. Different mechanisms by which IGs solve the 
problem of mRNA processing and nuclear export are discussed here, along with 
their possible impact on reporter studies.