Balsaminaceae and Marcgraviaceae are two closely related families of which the flower morphology is very different. A feature that both families have in common is the presence of petaloid struct- ures. Balsaminaceae have a characteristic petaloid sepal spur and Marcgraviaceae have petaloid nector beakers, but whether it con- cerns of a true synapomorphy is an open question. Because the combined function of A- and B-function genes controls the iden- tity of petals in model plants, we want to investigate whether an ectopic expression of these genes was present in the latest com- mon ancestor of Balsaminaceae and Marcgraviaceae. The existing classifications of Balsaminaceae are dated or restricted to a defined geographical region. Comparative sequence analysis will undoubtably contribute to Balsamincaceae.