Bengal cats from the first three filial generations of breeding (F1–G3) are considered "foundation cats" or "Early Generation" Bengals. The Early generation (F1–G3) males are frequently infertile. Therefore, female early generation Bengals of the F1, G2, and G3 are bred to fertile domestic Bengals. F1 hybrid Bengal females are fertile, thus they are used in subsequent, unidirectional back-cross matings to fertile domestic cat males. Some male Bengals produced viable sperm as early as the G2 back-cross generation: this is considered rare in the breeding communities, who regularly back-cross early generation females to late generation, fertile hybrid males. The infertility of male F1 Bengals is the reason why all subsequent generations of Bengal cats are characterized by the letter G as opposed to F (acquiring an F2 Bengal would imply two F1 parents which is impossible). As such, the generations of Bengal cats are F1, G2, G3, G4 and so on. Nevertheless, as the term was used incorrectly for many years, many people and breeders still refer to the cats as F2, F3 and F4 even though the term is considered incorrect.