I was looking at genome.ucsc.edu for chromosome 1 and realized that it was marked as "NNNNNN" sequences for the end of chromosome.

Why isn't it repeating as "TTAGGG" for these telomeric regions? Isn't N defined as undefined or error regions?

Thank you
See: https://www.biostars.org/p/66688/#66689

Truly complete sequence of human genome was just pre-published last month. https://www.biorxiv.org/content/10.1101/2021.05.26.445798v1 It will take a while before official genome build gets to that level of resolution.