Conversion of farnesyl pyrophosphate to squalene

Catalyzed by squalene synthase.

Dissociation of the PPi group on one of the molecules of farnesyl-PPi yields an allylic carbocation.

The double bond nucleophilically attacks the carbocation, forming a tertiary cation at C3 of the first farnesyl-PPi.

Loss of a proton from C1 gives the activated cyclopropane intermediate, presqualene-PPi.

Dissociation of the second pyrophosphate leads to rearrangement and formation of another tertiary carbocation intermediate.

Hydride transfer from NADPH completes the rearrangement, giving squalene.