Classical genetic studies show that gene conversion can favour some alleles over others. Molecular experiments suggest that gene conversion could favour GC over AT basepairs, leading to the concept of biased gene conversion towards GC (BGC(GC)). The expected consequence of such a process is the GC-enrichment of DNA sequences under gene conversion. Recent genomic work suggests that BGC(GC) affects the base composition of yeast, invertebrate and mammalian genomes. Hypotheses for the mechanisms and evolutionary origin of such a strange phenomenon have been proposed. Most BGC(GC) events probably occur during meiosis, which has implications for our understanding of the evolution of sex and recombination.