(A) A tissue that consists entirely of cells in which one of the Rp genes is present in only one copy (Rp+/-, shown in light blue) grows to a normal size. However, Rp+/- cells are outcompeted and eliminated by apoptosis when they are adjacent to wildtype cells (Rp+/+, orange). The loss of Rp+/- cells depends on a transcription factor called Xrp1: if cells lack one Rp allele and are also mutant for Xrp1 (Rp+/-, Xrp1-/-, shown in purple) they will not be eliminated. (B) The Rp genes are spread out over all four chromosomes of the fruit fly. About 65 of these genes (red dots) cause the Minute phenotype when only one copy is present (one red dot across from one white dot). When a heterozygous cell, in which any of these 65 genes is present in only one copy (Rp+/-), is located adjacent to a cell with a complete set of Rp genes (Rp+/+), this triggers the expression of Xrp1 and its binding partner Irbp18. The increased activity of this Xrp1/Irbp18 dimer is required for the effects of cell competition: decreased protein synthesis, slower growth and ultimately apoptosis in the Rp+/- cells.