UBQLN2 and neurodegenerative disease: This is a question that may provide strong clues about their relationship with neurodegenerative diseases; 3) whether different UBQLN4 group proteins have different affinities for different types of ubiquitinated proteins or even for particular types of ubiquitin chains, and, 4) whether there has been positive selection on the UBQLN2 gene, as suggested by the “free-ratio” model results (see above).