BX (backcross) is when a breeder crosses a hybrid back to one of its parents — usually the more desirable parent. The goal is to push the offspring's genetics closer to that parent while keeping selected traits from the other side.

BX1 is the first backcross (offspring × parent). BX2 is the offspring of BX1 × the same parent. BX3, BX4, etc. each push closer to the parent line. Each backcross roughly halves the contribution from the other parent.

Common use case: a breeder has a clone-only flavor (say Forum Cut GSC) they can't legally distribute. They cross it to a stable male to get S1 (selfed seeds) or BX1 with a different parent — and backcross until the seed line consistently expresses that Forum Cut profile.

BX is the standard tool for turning clone-only cuts into accessible seed lines.

What's the difference between BX and S1?+
BX uses two different parents (offspring × parent). S1 is a self-cross (a plant pollinated with its own reversed pollen).
How many BX generations do you need?+
Usually BX2 or BX3 is enough to lock in the target traits. More than that and the line risks losing diversity.