What are the coordinates of the extended major histocompatibility complex (xMHC) in GRCh38? Is there an accepted boundary of the extended major histocompatibility complex (xMHC) in human? Specifically, I am interested in the boundary coordinates for Genome Reference Consortium Human Build 38 (GRCh38 / hg38).
I'd like to use these coordinates to query MyGene.info to retrieve a list of genes in the xMHC. For example, King et al 2019 used the following definition:

we excluded drugs with nonhuman or xMHC gene targets. We considered xMHC to include HIST1H2AA and KIFC1 and all genes between them (Chromosome 6 25.7 Mb-33.4 Mb).

But I was curious whether there was an authoritative definition of the xMHC region in humans that would specify down the the base pair rather than just megabase pair.
Here is one definition of the classical MHC region: https://www.ncbi.nlm.nih.gov/grc/human/regions/MHC?asm=GRCh38.p13
which defines it as the 5mb region chr6:28510120-33480577 in GRCh38 coordinates.
The &quot;extended MHC&quot; defined here [1] is 7.2 mb and encompasses an additional &quot;extended Class I&quot; region upstream of the region given above. The xMHC is defined in [1] as the regions between HIST1H2AA and RPL12P1 genes, which would give you chr6:25726063-33400644 in Grch38. This is very similar to your HIST1H2AA - KIFC1 definition (chr6:25726063-33410226). Of course if you're trying to reproduce a result, probably better to use the original paper's definition.
[1] https://www.nature.com/articles/nrg1489