Seems to be an immortalized (telomerase*-transformed) cell line from a female fetus with near-complete homozygosity (https://sites.google.com/ucsc.edu/t2tworkinggroup/chm13-cell...).

* Telomerase is a reverse transcriptase that allows to achieve replicative immortality (https://academic.oup.com/hmg/article/9/3/403/715108).