When estrogen is present it can enter cells in the vicinity and it can bind to receptors that are deep inside cells.

雌激素能够进入到邻近的细胞中,并能够与细胞内部的受体相结合

耶鲁公开课 - 生物医学工程探索课程节选

So, it doesn't need to bind to a receptor on the surface of the cell in order to work because the molecule can actually enter the cell directly.

所以无需与细胞表面受体相结合来起效,因为这些分子能直接进入细胞

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For example, HIV enters cells of the immune system by binding to a receptor called CD4.

比如,HIV通过与CD4的受体结合,进入到免疫系统细胞中

耶鲁公开课 - 生物医学工程探索课程节选