DNA can be delivered into cells, usually by using a viral vector, to alter protein expression. This approach could be used to express proteins that inhibit reproduction by a variety of methods. Promising results have been observed using viral vectors to cause the expression of MIH in female mice[30] and in female cats.[31] Similarly, expression of antibodies against GnRH has been effective in mice,[32] but the levels achieved in cats were insufficient to be effective.[33] Immune responses to the expressed proteins may have limited the duration of the contraceptive effect.