*Class I ARE elements, like the c-fos gene, have dispersed AUUUA motifs within or near U-rich regions.
*Class II elements, like the GM-CSF gene, have overlapping AUUUA motifs within or near U-rich regions.
*Class III elements, like the c-jun gene, are a much less well-defined class—they have a U-rich region but no AUUUA repeats.
No real ARE consensus sequence has been determined yet, and these categories are based neither on the same biological functions, nor on the homologous proteins.