Sequencing from two extreme.

Sequencing method of a nucleic acid molecule, comprising the steps of: (a) hybridizing two or more sequencing primers with one or a plurality of individual chains of the nucleic acid molecule, in which all primers except one are reversibly blocked primers, (b) incorporating at least one base in the nucleic acid molecule by elongating polymerase from an unblocked primer, (c) blocking the additional elongation of said unblocked primer, (d) unlocking one of reversibly blocked primers, forming an unlocked primer, (e) repeat steps (b) through (d) until at least one of the reversibly blocked primers has been unlocked and used to determine a sequence by pyrophosphate sequencing.