This phenotypic mutant was identified in an ENU mutagenesis screen. A noncomplementation test with other Npc1 alleles showed that nmf164 represents an allele of Npc1. The mutation is an A to G transversion at coding nucleoide 3014 (c.3014A>G) in exon 20 of 25 exons, altering the corresponding amino acid from aspartate to glycine at codon 1005 (p.D1005G) in loop I of the protein between the eighth and ninth (of 13) transmembrane domains. Western blot analysis reveals that protein levels reduced to 10-15% of wild type.