I'm using biblatex and biber, and trying to modify the usual styles to include a field for the AMS MathReviews number. My BibTeX file (well, I guess it's a Biber file now) has `mrnumber` fields, and I've modified the relevant `.bbx` files, but when I run Biber, it doesn't do anything with the mrnumber field -- and so those never appear in the bibliography.

I manually modified the `.bbl` file (I put in `\field{mrnumber}{123456}`) and the MR number gets printed. So, in addition to modifying the `.bbx` files, how do I get Biber to pass the relevant fields into the `.bbl` file?