the role of alternative promoters in mammalian genomes

In [PMID: 14585616], the authors estimated 18% of all human genes (~2500 loci) have evidence for alternative promoter usage. They also listed recently published studies of genes with alternative promoters.

The paper mentioned many interested feature regarding promoters:
1. Indeed, the use of endogenous retroviral sequences and other transposable elements as transcriptional promoters, although often over looked, is not uncommon in the genome.

2. The simplest and most common scenario involves two or more promoters that produce transcripts with identical ORFs. From studies published to date, it appears that 60–80% of genes with alternative promoters are of this type.

3. Does alternative promoter simply represent an inherent ‘leakiness’ in transcriptional control of the genome? Perhaps the strongest evidence supporting the idea that complex control of genes is not simply a biological artifact is the extent to which complex promoter structures are conserved between species. The examples of homologous genes with multiple promoters that are conserved in humans and rodents suggests that, in many cases, selective pressure has acted to maintain this complex regulation of gene expression.