UBQLN2 and hereditary spastic paraplegia: UBQLN2 or Ubiquilin2 (sometimes referred to as Dsk2, PLIC-2, or CHAP1) is the most well-characterized of the five ubiquilins and perhaps the most well-studied UbL-UbA protein because mutations in the UBQLN2 gene have been identified in rare cases of X-linked juvenile and adult forms of ALS, ALS/FTD [111], and the atypical hereditary spastic paraplegia phenotype [112].