Monoecious and dioecious species produce unisexual flowers in which either male or female reproductive organs develop. These species have evolved a mechanism to selectively abort or arrest one of the sexes, a phenomenon named sex determination. On pages 481–493 of this issue, Kater et al. describe a set of floral homeotic mutants of the monoecious species cucumber, which provides novel information about the spatial control of sex determination. The homeotic mutants were generated by altering the expression of MADS box transcription factors, which resulted in mutants resembling the ABC classes of flower mutants. In male homeotic mutant flowers, carpels developed in the first (instead of sepals) and third whorl (instead of stamens) but they never developed in the restricted fourth whorl. Similarly, male tissue was observed in the second whorl of female flowers but never in the restricted third whorl. Interestingly, the outgrowth of nonreproductive organs is not prevented in these restricted floral whorls, as was observed in a class C mutant (female flower shown on the cover). The petals in the center of this female flower developed from the normally restricted third whorl, suggesting that the sex determination machinery is able to sense the reproductive identity of the developing organs in whorls 3 and 4. The authors suggest that the class C homeotic gene function is required for the position-dependent arrest of reproductive organs.