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I m looking for universal primers M13. I found different version. For instance :

  • M13 (-21) Forward TGTAAAACGACGGCCAGT
  • M13 (-40) GTTTTCCCAGTCACGAC

What's the meaning of (-21) and (-40 )

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This dates back to the 1980's when these M13 primers were first cloned.

Heidecker G, Messing J, Gronenborn B. 1980. A versatile primer for DNA sequencing in the M13mp2 cloning system. Gene 10(1):69-73.

In this paper, they discuss the construction of a primer for sequencing of the M13mp2 phage vector. An EcoRI/AluI digest releases a 92 bp fragment from a portion of the lac operon. This fragment contains what are now called the M13 forward -21 and -40 primers. I do not know who first named these primers but, counting from the centre of the EcoRI palindrome, I think it's clear where these names came from:

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