Mutations in the OA1 gene cause ocular albinism type 1 (OA1), an X-linked form of albinism affecting only the eye, with skin pigmentation appearing normal. To better understand the pathogenesis of this disease the time of onset and the pattern of expression of the mouse homolog of the OA1 gene were monitored during eye development. The localization of Oa1 mRNA was studied and compared with the expression of other genes involved in melanosomal biogenesis.The Oa1 expression pattern during eye development and after birth was analyzed by reverse transcription-polymerase chain reaction (RT-PCR) and in situ hybridization. Localization of Oa1 mRNA was compared with TYROSINASE: (TYR:), pink-eyed dilution (p), and Pax2 expression patterns.RT-PCR revealed that Oa1 expression began at embryonic day (E)10.5 and was maintained until adulthood. By in situ hybridization analysis Oa1 transcripts were detected in the retinal pigment epithelium (RPE) beginning at E10.5 in the dorsal part of the eyecup and in the same area where transcripts of other genes involved in pigmentation are found. Of note, the expression pattern of these genes was complementary to Pax2 expression, which was restricted to the ventral side of the optic cup. At later stages, expression of Oa1, TYR:, and p expanded to the entire RPE and ciliary body.Oa1 expression can be detected at early stages of RPE development, together with other genes involved in pigmentation defects. Oa1 is likely to play an important function in melanosomal biogenesis in the RPE beginning during the earliest steps of melanosome formation.