Odorant receptor (OR) genes can be classified into two types: fish-like class I OR genes and mammalian-specific class II OR genes. We have previously shown that Lhx2, a LIM-homeodomain protein, binds to the homeodomain site in the promoter region of mouse M71, a class II OR, and that a knockout mutation in Lhx2 precludes expression of all tested class II OR genes including M71. Here, we report that most class I OR genes, which are expressed in a dorsal region of the olfactory epithelium, are still expressed in Lhx2-deficient embryos. There are two exceptions: two class I OR genes, which are normally expressed in a more ventral region, are no longer expressed in Lhx2 mutant mice. Lhx2 is transcribed in olfactory sensory neurons irrespective of expression of class I or class II OR genes. Thus, a deficiency of Lhx2 has a differential impact on class I and class II OR gene expression.