What is the chromosomal mutation of XP (Xeroderma Pigmentosum)

I just need to know what chromosome it's on (i.e. 4, 11, 17) and what happens to it to cause XP (i.e. base-pair substition etc.)

Also...I'm in 9th grade biology. Nothing too advanced please!! Thanks:)

I'd like to make it easy on you, but there are several genes: DDB2, ERCC2, ERCC3, ERCC4, ERCC5, POLH, XPA. XPC. Of these, XPA (chromosome 9) and XPC (chromosome 3) are the most common.