Rett syndrome is due to a genetic mutation of the MECP2 gene. This condition is associated with mutations of the PTEN gene. One form is due to mutations of the glucokinase gene. OI caused by a mutation in the bone morphogenic protein 1 (BMP1) gene. Genetic tests are available for some of these gene variations. The bobtail is due to the mutation of a dominant gene. Prion proteins are encoded by the Prion Protein Gene (PRNP). OI caused by mutations in the TMEM38B gene. OI caused by mutations in the CREB3L1 gene. Schwannomatosis is caused by a mutation on the SMARCB1 gene. Spinal muscular atrophy is due to a genetic defect in the "SMN1" gene. Alleles are different forms of a gene. In humans, the gene CYP19, located on chromosome 15q21.1, encodes aromatase. Ataxia-telangiectasia is due to mutation in the "Atm" gene. There are several layers of regulation of gene expression.