I just ran into this in the annotated genome of Mycobacterium tuberculosis H37Rv:
3187030..3187611
/gene="mpt70"
/locus_tag="Rv2875"
/function="NOT REALLY KNOWN."
Someone at the Sanger Institute has a sense of humor, I think.
(For a little more information, the product of this gene is a protein known as Mpt70. Although the function of Mpt70 is unknown, it is an immunogenic protein, meaning that your immune system kicks up a fuss when exposed to it. This usually means it does something important, so we can be assured that people are studying it.)