The typical form of MLIV, caused by the absence of functional TRPML1 protein or its complete loss of function, is characterized by hypomyelinating leukodystrophy with brain iron accumulation and manifests with severely impaired psychomotor development and a gradual neurological decline paralleled by cerebellar neurodegeneration and neuroaxonal injury (Frei et al., 1998; Bonavita et al., 2003; Schiffmann et al., 2014). The gene discussed is MCOLN1; the disease is mucolipidosis type IV.