Complex sentence: TAM is more promiscuous than RAL in target sites like the uterus because of the relationship between D351 in the ER and the antiestrogenic side chain of the SERM.
Split into simpler sentences: TAM is more promiscuous than RAL in target sites like the uterus. This is because of the relationship between ER's amino acid in D351 and the antiestrogenic side chain of the SERM.