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The priorities are assigned to different substituents while finding an E and Z isomer.

If the two higher priority groups are on the same side of the double bond, then they are called Z-isomers and if the two higher priority groups are on the trans side of the doubly bonded atoms then they are termed E-isomer.

1. Assign the priority order to the groups on the basis of atomic number. The higher atomic number gets the highest priority.

2. If higher priority groups are on the same side, then it will be a Z isomer as shown below.

3. If higher priority groups are on the different side, then it will be an E isomer as shown below.

Tamoxifen slows the growth of some breast tumors by binding to estrogen receptors. The higher priority groups are on the same sides of the double bond so tamoxifen has theZ configuration.