Due to the distinct chemical and physical properties of the nucleotides in the DNA and some mutations being "silent" (that is, not altering gene expression), the DNA primary sequence may also be a phene. For example, A-T and C-G base pairs are differently resistant to heat (see also DNA-DNA hybridization). In a thermophilic microorganism, "silent" mutations may have an effect on DNA stability and thus survival. While being subject to evolution, natural selection affects the primary sequence directly in this case, with or without it being expressed.