Abstract: Fertility problems affect (1 in 10) couples in Western society, making it one of the most common serious health issues. Despite this, little is known about the causes of infertility, and thus patient counseling and treatment are suboptimal. With infertility being such a common problem, identification of any cause would impact on a large number of patients, allowing better counseling, clearer diagnoses and the possibility of making more informed choices (e.g. adoption vs. IVF treatment). The present invention provides methods to identify a cause of infertility in a subject based on the genotype of the subject, in particular, by evaluating the status of the gene encoding FK506 binding protein-like (FKBPL). In particular, the present invention relates to use of the status of the gene encoding FK506 binding protein-like for identification of a cause of an infertile phenotype in a subject.