Recent analysis of the contribution of replication slippage to genome evolution shows that it has played a significant role in all species from eubacteria to humans. The overall level of repetition in genomes is related to genome size and to the degree of repetition that can be measured within individual ribosomai RNA genes, suggesting that the entire genome accepts simple sequences in a concerted manner when its size increases. Although coding sequences accept simple sequences much less readily than non-coding sequences, they accept some repeats, particularly (CAG)n, preferentially. This may have consequences for the evolution of the genes involved in trinucleotide expansion diseases and the transcriptional networks of which they may form a part.