this refers to a permanent change to the DNA sequence to produce something different. Everyone with MLIV has two-alleles (see above) which code for MCOLN1: you need to have mutations in both of those alleles in order to have MLIV. There are many different mutations that can cause MLIV in people. Some combinations of mutations cause a more severe form of the disease because they appear to leave some TRPML1 (see below) active in the cells. Some mutations appear to leave no TRPML1 available at all in the body and this seems to account for the most severe forms of MLIV.