Among various photosensitive disorders, photosensitive genetic diseases are noticeable in that physicians play a key role in early diagnosis. Xeroderma pigmentosum (XP) and erythropoetic protoporphyria (EPP) are focused on this issue as photosensitive genetic diseases. XP is characterized by the proneness to skin cancers of sun-exposed area from younger age. XP-A through G and XP-V are caused by the deficiency in nucleotide excision repair (NER) and by translesional synthesis, respectively. NER is closely related to the transcription and the neurological features developed in some XP patients are attributed to the transcription abnormalities. As for EPP, it was shown that the combination of mutation in FECH in one allele and the low expression type SNP in the other allele cause the apparent clinical phenotype.