Bietti crystalline corneoretinal dystrophy (BCD) is a relatively rare form of retinal degeneration, first reported by Bietti in 1937. The typical changes are yellowish-white glittering crystalline deposits in the retina with retinal pigment epithelium and choroidal atrophy, and in some patients, crystalline deposits may be seen in the superficial corneal stroma near the corneal limbus. BCD is not common worldwide, but patients are more common in China and Japan. Professor Hu Danning speculated that the frequency of mutations in the causative gene in the Chinese population is 0.005, and the population prevalence is 1/24,000 (Hu D, 1983), with no gender differences. Clinical manifestations] Symptoms: Most patients are 20-40 years old and both eyes are involved. The clinical manifestations are progressive vision loss, or night blindness, or both. There are also patients who have no conscious symptoms and are only detected by fundus examination. Color vision is normal in the early stages, but in the later stages, red-green or total color blindness may occur. The main clinical categories are limited or diffuse (Wilson DJ et al., 1989): limited: retinal lesions are mainly confined to the posterior pole and patients often present with paracentral dark spots; diffuse: extensive RPE lesions and pigmentation, patients often present with peripheral visual field defects, decreased central vision and ERG changes. Slit lamp microscopy: in a few cases (about one-third), crystalline, yellowish-white, fine punctate deposits can be seen in the superficial stroma of the corneal rim. Funduscopic examination: Early stage: yellow, crystalline, round, fine or irregularly shaped, larger shimmering bright spots are scattered in the posterior pole of the retina. Most of the crystalline lesions are located in the RPE cell layer of the retina, and some of them are located in the neuroretina. Progressive stage: As the disease progresses, the retinal pigment epithelium and choroidal capillaries gradually atrophy. At this stage, osteocyte-like or irregular-shaped pigmentation is visible outside the atrophic area, and crystalline material is rarely present in the atrophic area, and exposed large choroidal vessels are visible. Late stage: Extensive retinal pigment epithelium and choroidal capillary atrophy, with occasional crystalline deposits. The optic papilla and retinal vessels are normal in the early stage, with optic nerve atrophy and retinal artery thinning in the late stage. Yuzawa et al. divided BCD into three clinical stages (Yuzawa M et al., 1986): Stage I: posterior pole RPE cell atrophy with fine, uniform, white crystalline deposits; Stage II: enlarged RPE atrophy area, choroidal capillary atrophy within the posterior pole RPE atrophy area; Stage III: extensive atrophy of RPE and choroidal capillaries. Color vision: lesions involving the macular area may be accompanied by color vision abnormalities. Visual field: visual field defects within the atrophic area. Focal atrophy is mostly manifested as a central dark spot, while diffuse atrophy is dominated by peripheral visual field defects. ERG: Depending on the extent of the lesion, the ERG may show mild abnormalities, moderate abnormalities, severe abnormalities or even no waveform. When the retinal pigment epithelium and choroidal capillary layer are involved, the ERG and visual field are altered accordingly. Case reports of RPE and choroidal capillary atrophy with a normal ERG have also been seen. FFA: Early in the lesion, posterior pole translucent fluorescence is seen, with crystalline deposits causing fluorescence masking; as the disease progresses, window-like translucent fluorescence is seen in areas of retinal pigment epithelial atrophy; in areas of retinal pigment epithelial and choroidal capillary atrophy, diffuse hypofluorescence and exposure of large choroidal vessels are seen. Mataftsi A et al. classified BCD FFA images into the following three levels (Mataftsi A et al., 2004): Early stage: diffuse RPE changes (pretzel-like) with diffuse small or medium-sized hypofluorescent spots (corresponding to RPE pigment changes or choroidal capillary filling defects) in the posterior pole and mid-peripheral retina; Progressive stage: extensive RPE changes in the posterior pole and mid-periphery Extensive atrophy of the RPE-choroidal capillary complex is visible in the retina, with better preservation of the peripheral part of the optic papilla and the macula; Severe stage: complete atrophy of the posterior pole. [Differential diagnosis] Patients with advanced lesions present with severe visual loss, extensive retinal choroidal atrophy, reduced crystalline lesions, and no waveform in ERG, which need to be differentiated from chorioretinopathy and retinitis pigmentosa. It is known that 80-90% of patients with BCD are caused by mutations in the CYP4V2 gene, which is located at 4q35.2. [Treatment and Prognosis] There is no effective treatment for this disease. Transgenic therapy is a promising treatment for the disease.