chromosome name confusion

Question

[irfanwustl]

I have downloaded a GENCODE track from UCSC genome browser which has the list of genes and their genomic positions. However, for some genes, I am confused with the chromosome name. For example, I have seen chr15_KI270905v1_alt as a chromosome. I know there is chromosome 15 but what does chr15_KI270905v1_alt mean?

[Ram]

This might be useful: https://gatk.broadinstitute.org/hc/en-us/articles/360041155232-Reference-Genome-Components

_alt "chromosomes" are called alt contigs. These are not really chromosomes; they're contigs (contiguous sequences). The reference genome is made of a bunch of contigs, of which the principal ones are chromosome-wide contigs.