From comparative genomic studies, at least 3–8% of bases are under purifying (negative) selection 4–11 indicating that these bases may potentially be functional. We previously found that 60% of mammalian evolutionarily constrained bases were annotated in the ENCODE pilot project, but also observed that many functional elements lacked evidence of constraint2, a conclusion substantiated by others52–54. The diversity and genome-wide occurrence of functional elements now identified provides an unprecedented opportunity to further examine the forces of negative selection on human functional sequences.
