Organization of two Drosophila odorant-binding protein gene clusters in chromosomal regions 56E-F and 57A. The upper panel depicts a cluster of nine Obp genes and one odorant receptor (Or) gene located in chromosomal region 56E-F. The length of each arrow corresponds to the length of the coding sequence, with arrowheads indicating direction of transcription. Note thatOr56a is located near the center of the Obp56 gene cluster. There are also two other nonolfactory genes in this region that have been omitted for clarity. (A predicted chaperone protein gene lies between Obp56f and Or56a, and a predicted cell-adhesion protein gene lies between Obp56h andObp56i.) The lower panel depicts a cluster of fiveObp genes located in chromosomal region 57A. For clarity, six other nonolfactory genes in this region (located betweenObp57c and Obp57d) have been omitted.