Long-read sequencing is required for precision diagnosis of incontinentia pigmenti.

Incontinentia pigmenti (IP) is caused by loss-of-function variants in IKBKG, with molecular genetic diagnosis complicated by a pseudogene. We describe seven individuals from three families with IP but negative clinical genetic testing in whom long-read sequencing identified causal variants, including one family with the common exon 4-10 deletion not identified by conventional clinical genetic testing. Concurrent methylation analysis explained disease severity in one individual who died from neurologic complications, identified a mosaic variant in an individual with an atypical presentation, and confirmed skewed X chromosome inactivation in an XXY individual.