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Glossary Term
Glossary Term Null
Definition In mathematics, a set with no members or of zero magnitude. If a field has a value of null, it means that the value is unknown. A null value is not the same as a value of zero. (To appreciate the difference, consider the terms "free" and "priceless." If something is free, it has a price of zero. If something is priceless, it has no known price. The difference between null and zero can be crucial; for example, when calculating the average value of a field among many records where one row contains a zero, the zero gets factored into the average. If the field has a null value, it does not get factored into the average.)

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Mouse Genome Database (MGD), Gene Expression Database (GXD), Mouse Models of Human Cancer database (MMHCdb) (formerly Mouse Tumor Biology (MTB)), Gene Ontology (GO)
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MGI 6.22
The Jackson Laboratory