'If G-quadruplexes form so readily in vitro, Nature will have found a way of 
using them in vivo' (Statement by Aaron Klug over 30 years ago).During the last 
decade, four-stranded helical structures called G-quadruplex (or G4) have 
emerged from being a structural curiosity observed in vitro, to being recognized 
as a possible nucleic acid based mechanism for regulating multiple biological 
processes in vivo. The sequencing of many genomes has revealed that they are 
rich in sequence motifs that have the potential to form G-quadruplexes and that 
their location is non-random, correlating with functionally important genomic 
regions. In this short review, we summarize recent evidence for the in vivo 
presence and function of DNA and RNA G-quadruplexes in various cellular pathways 
including DNA replication, gene expression and telomere maintenance. We also 
highlight remaining open questions that will have to be addressed in the future.