This document relates to methods and materials for determining whether or not a horse contains a Grey allele. For example, diagnostic methods such as nucleic acid-based detection methods and materials such as nucleic acid probes and primer pairs that can be used to determine whether or not a horse contains a duplication in intron 6 of STX17 nucleic acid are provided. This document also relates to methods and materials for treating a mammal having or being likely to develop cancer (e.g., benign, malignant, or metastatic cancer). For example, methods and materials for treating cancer in a mammal by administering an agent having the ability to reduce expression of an STX17 polypeptide and/or an NR4A polypeptide (e.g., an NR4A1, NR4A2, or NR4A3 polypeptide) in the mammal are provided.