Выражение встречается в следующем контексте:
Based on extensive testing among a representative population, no discoloration is expected from regular usage of blend-a-med. However in the unlikely event a slight discoloration occurs in select individuals. If pressed: What are the characteristics of these select individuals? Individuals with areas in their mouth that is hard to reach with a toothbrush. Individuals who have a high susceptibility to stain, smokers, heavy tea and coffee drinkers, and those who might not practice good oral hygiene could be susceptible to stain.

Заранее спасибо