In a case-control study, an odds ratio (OR) of less than 1 for a single nucleotide polymorphism (SNP) site indicates that the SNP is associated with a decreased risk of the disease or condition being studied. The closer the OR is to 0, the stronger the protective effect of the SNP. It is important to note that an OR of less than 1 does not necessarily mean that the SNP is causally related to the disease, as there may be other factors at play.