DNA sequence alignment is a prerequisite to virtually all comparative genomic analyses, including the identification of conserved sequence motifs, estimation of evolutionary divergence between sequences, and inference of historical relationships among genes and species. While it is mere common sense that inaccuracies in multiple sequence alignments can have detrimental effects on downstream analyses, it is important to know the extent to which the inferences drawn from these alignments are robust to errors and biases inherent in all sequence alignments. A survey of investigations into strengths and weaknesses of sequence alignments reveals, as expected, that alignment quality is generally poor for two distantly related sequences and can often be improved by adding additional sequences as stepping stones between distantly related species. Errors in sequence alignment are also found to have a significant negative effect on subsequent inference of sequence divergence, phylogenetic trees, and conserved motifs. However, our understanding of alignment biases remains rudimentary, and sequence alignment procedures continue to be used somewhat like benign formatting operations to make sequences equal in length. Because of the central role these alignments now play in our endeavors to establish the tree of life and to identify important parts of genomes through evolutionary functional genomics, we see a need for increased community effort to investigate influences of alignment bias on the accuracy of large-scale comparative genomics.The relative positions of nucleotides within the same gene in different species and in duplicated genomic regions are disturbed by insertion and deletion of stretches of DNA over evolutionary time. This leads to differences in the length of the homologous regions in the genome, with more distant relatives having a higher likelihood of sequence length difference. A comparison of lengths of genome segments spanning protein-coding genes in human and mouse shows the extent of the effect of evolution by insertions and deletions (Fig. 1). The lengths of noncoding orthologous sequences have also evolved substantially after divergence over 90 million years ago. A grand challenge in comparative genomics is to line up these bases by inserting gaps in sequences, because genomic analyses must be based on comparisons between bases at positions (sites) that coincided in a common ancestor. The task is to re-establish (estimate) the ancestral site-wise homology obfuscated by the insertiondeletion and substitution processes. Naturally, this operation has come to be known as "alignment," and the resulting set of sequences, all of which are the same length (taking gaps in to account), is also called an alignment (Fig. 2). We can distinguish between "pairwise" alignments, in which sequences, even if they are part of a larger set, are aligned only in pairs, and "multiple" alignments, in which more than two sequences are aligned simultaneously.Alignment procedures may also be classified as either "global" or "local...