We have investigated the hybridization properties of DNA-gold nanoparticle conjugates and have discovered that the hybridization properties are dramatically affected by controlling various synthetic and environmental conditions. We have further demonstrated that moderate DNA loading instead of high loading per nanoparticle significantly enhances the hybridization rates of DNA-gold nanoparticle conjugates, which allows one to precisely design their hybridization properties to distinguish a single-nucleotide polymorphism (SNP). A diagnostic application for the colorimetric detection of an SNP associated with a mutation in the breast cancer gene BRCA1 has been carefully designed and demonstrated.