a method of determining the length of a polynucleotide target is provided . with this method , a target is first hybridized to an array of first probes having different , determined lengths , resulting in the formation of duplexes between the polynucleotide target and the first probes . these duplexes have a single stranded section of target if the target is longer than the first probe it is in a duplex with . next , a second probe having a determined length is hybridized to these duplexes . if the length of the target is greater than the length of the first probe it is displaced during this hybridization step by the process of branch migration . in contrast , if the length of the target is less than or equal to the length of the first probe , it is not displaced . thus , the length of the polynucleotide target can be determined .