The chromosomal locus is at 3p25. The "BTD" gene has 4 exons of lengths 79 bp, 265 bp, 150 bp and 1502 bp, respectively. There are at least 21 different mutations that have been found to lead to biotinidase deficiency. The most common mutations in severe biotinidase deficiency (<10% normal enzyme activity) are: p.Cys33PhefsX36, p.Gln456His, p.Arg538Cys, p.Asp444His, and p.[Ala171Thr;Asp444His]. Almost all individuals with partial biotinidase deficiency (10-30% enzyme activity) have the mutation p.Asp444His in one allele of the "BTD" gene in combination with a second allele.