What change to the genetic code causes the niemann-pick disease?

Niemann Pick Disease: is a group of metabolic disorders with autosomal recessive inheritance. Each child of 2 carriers has a 25% chance of inheriting the defective gene from both parents & having the disease. Mutations of the SMPD1 gene on Chromosome 11p.15.4 cause Types A & B. Mutations of the NPC1 gene on Chr. 18q11.2 cause types C1 & D. A mutant NPC2 gene on Chr. 14q24.3, causes Type C2. See nnpdf.org. for more.