What is a proximal deletion breakpoint?
Let's break down the phrase as one of the commenters mentioned:

&quot;proximal&quot;: in the context of chromosomal position, this means closer to centromere, whereas &quot;distal&quot; means further from the centromere (the middle part of the chromosome).
&quot;deletion&quot;: referring to the genes that were deleted as a result of the chunk of the 21st chromosome being missing.
&quot;breakpoint&quot;: points at which the chromosomes was broken leading to the deletion.

Putting it all together, this says that the deletion breakpoint closer to the centromere falls within the NCAM2 gene, whereas the deletion breakpoint further from the centromere falls within the GRIK1 gene, with the other 17/19 genes falling between the two.
To verify this interpretation, we can use the USCS Genome Browser as mentioned. We can see that if we compare the position of the NCAM2 gene and the GRIK1 gene on the 21st chromosome, the former is closer to the centromere than the latter. We can also do a spot check to verify that ADAMTS1 one of the other 19 deleted genes, lies between the two of them.