 In RNA viruses , homologous recombination can be either precise or imprecise . In the precise type of RNA @-@ RNA recombination , there is no difference between the two parental RNA sequences and the resulting crossover RNA region . Because of this , it is often difficult to determine the location of crossover events between two recombining RNA sequences . In imprecise RNA homologous recombination , the crossover region has some difference with the parental RNA sequences – caused by either addition , deletion , or other modification of nucleotides . The level of precision in crossover is controlled by the sequence context of the two recombining strands of RNA : sequences rich in adenine and uracil decrease crossover precision . 
