This domain is considered a double homeodomain, termed HOCHOB, present in the C. elegans genome. Family members include CEH-91 and CEH-93 that share extended sequence similarity with each other upstream of their typical HDs (Homeodomains). CEH-92, another family member, has three copies of this domain. The domain consists of two divergent HDs that are separated by a linker of about 17 residues. The linker has a number of conserved positions, two of which are cysteine residues suggesting that they could be involved in metal binding. Hence, the name HOCHOB (Homeobox-cysteine loop-homeobox). Furthermore, there are two conserved histidine residues, one in each HD (in CEH-91 displaced by two positions), and there is also a conserved aspartic acid. It is speculated that the HOCHOB domain is an evolutionary novelty that is derived from two HDs and may have gained metal-binding capacity [1].

1: The Homeobox Genes of Caenorhabditis elegans and Insights into Their Spatio-Temporal Expression Dynamics during Embryogenesis. Hench J, Henriksson J, Abou-Zied AM, Luppert M, Dethlefsen J, Mukherjee K, Tong YG, Tang L, Gangishetti U, Baillie DL, Burglin TR; PLoS One. 2015;10:e0126947. PMID:26024448