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Mutations in the PISTILLATA (PI) gene of Arabidopsis thaliana cause homeotic conversion of petals to sepals and of stamens to carpels. It is thus classed as a B function floral homeotic gene and acts together with the product of the other known B function gene, APETALA3 (AP3). We have cloned PI and determined the time and places of its expression in developing flowers. Surprisingly, the initial patterns of PI and AP3 expression are different. By positive regulatory interactions between PI and AP3, later expression patterns are coincident or nearly coincident. The pattern of PI expression also depends on the activity of the floral development genes APETALA2 and SUPERMAN and on the activity of PI itself. The PI and APETALA3 proteins specifically associate in solution and so may act together in regulating PI and other genes.