Disclosed is a kit useful for determining the number of repeat units in a repeat region of a target nucleic acid comprising: a plurality of different-sequence primers, each containing (i) a target binding segment and (ii) a tag segment having a nucleotide sequence that uniquely identifies the target binding segment, a first primer extension reagent; and a second primer extension reagent, wherein at least one of the first or second primer extension reagents includes an extendible nucleotide having a photodestructible label attached thereto.