Cobalt-precorrin-5B (C<sup>1</sup>)-methyltransferase (), cobalt-precorrin-6A synthase, CbiD (gene)) is an enzyme with systematic name S-adenosyl-L-methionine:cobalt-precorrin-5B (C<sup>1</sup>)-methyltransferase. This enzyme catalyses the following chemical reaction
This enzyme catalyses the C-1 methylation of cobalt-precorrin-5B in the anaerobic pathway of adenosylcobalamin biosynthesis in bacteria such as Salmonella typhimurium, Bacillus megaterium, and Propionibacterium freudenreichii subsp. shermanii.