Probability of a homologous match across the monocot–eudicot divide, as a function of cDNA position, relative to the start of the coding region. TblastN searches were conducted in both directions, rice-to-arabidopsis andarabidopsis-to-rice. The cDNA data were divided into two equally populated groups, based on the size of the coding region, to emphasize that the reduced probability at the 5′-end is a position effect, not a gene size effect.