Confusion about a gene's description
The FAM symbols are defined in the HUGO Gene Nomenclature Committee guidelines. 

The FAM symbol is an anonymous and temporary identifier that is given to groups of poorly characterised genes which share more than 40% amino acid sequence identity (see Section 4.1). 

There are two human genes/proteins in the sequence similarity 166 group; FAM166A and FAM166B. These have some sequence similarity (more than 40%) but  function of this similar sequence is currently unknown. Once further characterisation has been performed for any FAM group they will likely receive more meaningful names.