The News Feature 'The Genome Finishers' ( Nature 462, 843–845; 2009) incorrectly states that a gap in the reference sequence of chromosome 4 was a deletion flanked by large gene duplications. The gap was an assembly error caused by attempting (and failing) to merge two alternative versions of gene sequence, which then erroneously appeared in the reference as large duplications in the UGT2B17 region.