We next looked for differences in cell death or proliferation in vivo using one of the severe progeroid NER syndromes, XPCS/XPA, in comparison to single mutant (XPCS and XPA) and heterozygous (control) littermates at postnatal days 5, 12, and 15 coincident with growth retardation and ataxia but prior to signs of weight loss or morbidity. Here, XPA is linked to cerebellar ataxia.