We describe the localization of the gene encoding reticulocalbin, a Ca 2+ -binding protein of the endoplasmic reticulum, on human chromosome 11p13 midway between the WT1 and the PAX6 genes and show that it is hemizygously deleted in WAGR individuals. The mouse reticulocalbin gene is also shown to map to the region of conserved synteny on mouse chromosome 2 and to be deleted in the Small eye Harwell ( Sey H ) mutation. Loss of the reticulocalbin gene could contribute to the early lethality of Sey H and Sey Dey homozygotes.