Genes without protein products: Is H19 the norm or the exception?

The increasing number of RNA polymease II transcripts without any apparent open reading frame has increased our awareness that gene functions can be s elected for without involving a protein product. By using the H19 gene as a point of reference, we highlight here several common features among non-co ding genes, such as their antisense position in subchromosomal expression d omains which are often genomically imprinted. We also discuss the need to c ritically examine the translatability of transcripts which are considered n on-coding. Finally, we present a model to explain the origin of non-coding genes.