<p>Hello,</p>

<p>I would like to know how the string of the field "LOCUS" on a Genbank (and other similar) entry is formed, and how to read its information. For example: what does NM stand for? What about the XR or XM entries? How do the number following the letters is obtained?</p>

The meaning of the LOCUS field is described [here](http://www.ncbi.nlm.nih.gov/Sitemap/samplerecord.html#LocusB)

The NM and XM entries, etc., are RefSeq identifiers rather than GenBank identifiers. RefSeq is effectively a well-curated subset of GenBank.

[This page](http://www.ncbi.nlm.nih.gov/RefSeq/key.html) explains the meanings of the various prefixes.