GeneCards genes have unique, informative and stable GeneCards identifiers (GC ids), provided by the GeneLoc Algorithm.
The id begins with GC, which is followed by the chromosome number (where '00' indicates unknown chromosome and 'MT' indicates the mitochondria), 'P' or 'M' for orientation (Plus or Minus strand), and approximate kilobase start coordinate.
For example: OXA1L, with GC id GC14P022766 is on chromosome 14 on the plus strand, starting at 22766 kilobases
Genes that are currently placed on a specific chromosome, but whose exact location on the chromosome is not yet known, receive a modified GC id, consisting of the chromosome and strand information, followed by a number, which indicates uncertain location, followed by a letter representing the specific contig containing the gene, and the gene's kilobase position on that contig.
For example: FAM231C, with GC id GC01P6B0035 is on chromosome 1 on the plus strand of contig NT_187368, starting at 35 kilobases
Genes located on the alternative reference sequences (haplotypes - see NCBI for a full explanation) have a special GC id made up of the chromosome and strand information, followed by a letter, and the gene's approximate kilobase start coordinate.
For example: KIR2DS5, with GC id GC19MA00037 is on chromosome 3 on the minus strand of ALT_REF_LOCI_18, starting at 37 kilobases
Genes whose positional information includes only the chromosome need a further modified GC id, which includes the chromosome number, followed by 'U9', indicating lack of strand and positional information, followed by five digits, assigned sequentially.
For example: GUK2, with GC id GC01U990078 is on chromosome 1. Its strand and position are currently unknown.
If an id needs to change in future versions because the previously reported position is refined, the superseded id remains associated with the gene, along with the new one, so it cannot be assigned to any other gene, and so that users can still find the gene by that id.