DNAH9, like other genes encoding axonemal dynein heavy chains, is a very large gene containing 69 coding exons encoding a 4,486-amino-acid protein. Consistent with laterality defects in DNAH9 mutant individuals, we show by in situ hybridization that Dnah9 is expressed in pit cells of the mouse embryonic ventral node (Figure 2C), indicating a role of DNAH9 in left-right patterning of the body axis similar to the ODA β-HC paralog DNAH11 and ODA γ-HC DNAH5.6, 28 During evolution, the gene encoding the ODA β-HC duplicated late during metazoan evolution at the origin of the Chordata49, 50 to produce three genes encoding ODA β-HCs in mammals. In human respiratory cilia, there are two different orthologs of the Chlamydomonas ODA β-HC, DNAH9 and DNAH11 (Figure S20), with distinct axonemal localization and most probably distinct functions. During metazoan evolution a third ODA β-HC emerged, DNAH17, which shows a distinct tissue distribution and seems to be important for sperm motility based on analyses of publicly available data bases for gene and protein expression (The Human Protein Atlas; GEO Profiles).