What is the characteristic finding in Fragile X Syndrome related to gene mutations?

Fragile X Syndrome is primarily associated with mutations in the FMR1 gene, which is located on the X chromosome. The characteristic finding in this condition is the presence of an expansion of CGG trinucleotide repeats in the FMR1 gene. In individuals who are affected by Fragile X Syndrome, the number of CGG repeats is significantly higher than the normal range, which typically consists of 5 to 44 repeats. When the number of repeats exceeds 200, it leads to methylation of the gene and loss of gene expression, causing the features associated with Fragile X.

The other answer choices do not pertain to Fragile X Syndrome specifically. The 4 bp deletion in a promoter, as well as the A1555G mutation and the c.940A>G mutation, are associated with other genetic conditions or mechanisms but do not describe the hallmark genetic change that characterizes Fragile X Syndrome. This specificity of the CGG repeat mutation in the FMR1 gene underscores its critical role in diagnosis and understanding the pathology of the syndrome.