Length determination of nucleic acid repeat sequences by discontinuous primer extension

1. 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.

2. The kit of claim 1 , which further includes an addressable array of immobilized, different tag complements, wherein each different tag complement contains a sequence that is complementary to one of said primer tag segments, under conditions effective to hybridize the tag segments to corresponding tag complements on the support.

3. The kit of claim 1 wherein the label comprises a fluorescent molecule.

4. The kit of claim 1 wherein the label is attached to the extendible nucleotide through a cleavable linker.

5. The kit of claim 1 , wherein the second primer extension reagent includes a nucleotide terminator having a label attached thereto.

6. The kit of claim 5 , wherein the nucleotide terminator comprises a label selected from the group consisting of a fluorescent molecule and a chemiluminescent molecule.

7. The kit of claim 5 , wherein the nucleotide terminator comprises a photodestructible label.

8. The kit of claim 1 , further comprising a polymerase.

9. The kit of claim 8 , wherein the polymerase is Taq DNA polymerase.