This family consists of various proteins involved in cytochrome c assembly from mitochondria and bacteria; CycK from Rhizobium, CcmC from E. coli and Paracoccus denitrificans and orf240 from wheat mitochondria. The members of this family are probably integral membrane proteins with six predicted transmembrane helices. It has been proposed that members of this family comprise a membrane component of an ABC (ATP binding cassette) transporter complex. It is also proposed that this transporter is necessary for transport of some component needed for cytochrome c assembly. One member CycK contains a putative heme-binding motif, orf240 also contains a putative heme-binding motif and is a proposed ABC transporter with c-type heme as its proposed substrate. However it seems unlikely that all members of this family transport heme nor c-type apocytochromes because CcmC in the putative CcmABC transporter transports neither. CcmF forms a working module with CcmH and CcmI, CcmFHI, and itself is unlikely to bind haem directly.