Instead of searching across all of the NCBI databases, start your search in the Gene database. Note some common search examples that are provided on the home page of the database. A search limited to the gene symbol and organism will narrow your search results to a single record or a few records. Here is a search example:
MTR[symbol] AND Mus musculus[organism]
You may be getting more than one entry (example) if there are shared symbols (known as aliases) between genes. (For example, the mouse MTR symbol is the official symbol for 5-methyltetrahydrofolate-homocysteine methyltransferase, but it also serves as an alias for the Terc gene.)
To obtain the best available sequences for the gene, select the link to the NCBI Reference Sequences (RefSeq) from the Table of contents on the right side of the Full report of a gene record (example). The link will take you to the section of the Gene record that reports RefSeqs for the gene. You can read more about the NCBI RefSeq project here.
- To access transcript RefSeq records, use the transcript accessions links (the accession formats are NM_ or XM_ for mRNAs, and NR_ or XR_ for non-coding transcripts). The record display formats are GenBank, FASTA, and Graphics.
- To access protein RefSeq records, use the protein accessions links (the formats are NP_ or XP_). The record display formats are GenBank, FASTA, and Graphics.
- To access genomic RefSeq records, use the genomic accessions links for the gene (the formats examples are NC_ or AC_). The record display formats are GenBank, FASTA, and Graphics.
See this table to get additional information about RefSeq accession formats.
Please see more details on accessing a genomic sequence for a gene.