(1) Researchers . . . used cavity-free teeth that were extracted because of tooth or gum disease. (2) And don't rush to brush your teeth right after consuming a bottle or two of sports drinks, because that would exacerbate enamel damage. 'Because' is a very common word. It states the reason for something. For example, 'I want to see Constantine because I like Keanu Reeves.' Take a look at (1) and (2): both use the word 'because', however, one is followed by 'of' while the other is not. We use 'because of' when the reason that follows is a noun or noun phrase. In (1), the reason - tooth or gum disease - is a noun phrase. When the reason that follows 'because' is a complete sentence, we simply use 'because', without 'of'. In (2), the reason is a complete sentence: 'that [rushing to brush your teeth right after consuming a bottle or two of sports drinks] would exacerbate enamel damage'.