DNA sequence length in numbers of base pairs.

1 kbp (kilo base pairs) = 1,000 bp (base pairs)
1 Mbp (mega base pairs) = 1,000,000 bp
1 Gbp (giga base pairs) = 1,000,000,000 bp

Alternatively, kb, Mb, and Gb is sometimes used, but kbp, Mbp, and Gbp is often preferred to avoid confusion with gigabyte (GB) of computer disk space.

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http://en.wikipedia.org/wiki/Base_pair