NCBI Taxonomy identifier for the gene source organism.
Reference:
Wheeler DL, Chappey C, Lash AE, Leipe DD, Madden TL, Schuler GD,
Tatusova TA, Rapp BA (2000). Database resources of the National
Center for Biotechnology Information. Nucleic Acids Res 2000 Jan
1;28(1):10-4
Benson DA, Karsch-Mizrachi I, Lipman DJ, Ostell J, Rapp BA,
Wheeler DL (2000). GenBank. Nucleic Acids Res 2000 Jan 1;28(1):15-18.