There are multiple definitions:

1. midbrain-hindbrain boundary(English)

   • a signaling center that patterns the adjacent midbrain and rostral hindbrain and forms at the junction of two gene expression domains in the early neural plate: an anterior otx2/wnt1 positive domain and a posterior gbx/fgf8 positive domain; aka isthmic organizer
   • Brain (dys)function, Anatomy
   • https://doi.org…nana.2017.00064

2. middle hepatitis B [surface glycoprotein](English)

   • LHB, SHB
   • Hepatology
   • https://doi.org….1111/liv.14298

3. mycorrhiza helper bacteria(English)

   • many bacterial strains have been reported to promote either endo- or ectomycorrhizal symbioses
   • Bacteriology
   • https://doi.org…fpls.2014.00579