BACKGROUND:The flower gene has been previously linked to the elimination of slow dividing epithelial cells during development in a process known as "cell competition." During cell competition, different isoforms of the Flower protein are displayed at the cell membrane and reveal the reduced fitness of slow proliferating cells, which are therefore recognized, eliminated, and replaced by their normally dividing neighbors. This mechanism acts as a "cell quality" control in proliferating tissues. RESULTS:Here, we use the Drosophila eye as a model to study how unwanted neurons are culled during retina development and find that flower is required and sufficient for the recognition and elimination of supernumerary postmitotic neurons, contained within incomplete ommatidia units. This constitutes the first description of the "Flower Code" functioning as a cell selection mechanism in postmitotic cells and is also the first report of a physiological role for this cell quality control machinery. CONCLUSIONS:Our results show that the "Flower Code" is a general system to reveal cell fitness and that it may play similar roles in creating optimal neural networks in higher organisms. The Flower Code seems to be a more general mechanism for cell monitoring and selection than previously recognized.