Vesicle-associated membrane protein-associated protein B / C, also known as VAMP-B/VAMP-C, VAMP-associated protein B/C, VAP-B/VAP-C and VAPB, is a single-pass type IV membrane protein which belongs to the VAMP-associated protein ( VAP ) family. VAPB contains one MSP domain. VAPB may play a role in vesicle trafficking. VAPB forms a heterodimer with VAPA. VAPB interacts with VAMP1 and VAMP2. Defects in VAPB are the cause of amyotrophic lateral sclerosis type 8 ( ALS8 ) which is a familial form of amyotrophic lateral sclerosis, a neurodegenerative disorder affecting upper and lower motor neurons and resulting in fatal paralysis. Defects in VAPB are also a cause of spinal muscular atrophy autosomal dominant Finkel type ( SMAF ) which is characterized by proximal muscle weakness that begins in the lower limbs and then progresses to upper limbs.