The frequency of subfertile TB stallions that carry the FKBP6 A/A-A/A genotype approaches 1–3% of the TB breeding stallion population (four out of 150 stallions evaluated in Central Kentucky, USA, for the presence of the susceptibility genotype8; and seven out of 1128 stallions of various breeds evaluated during 17 years at a reference laboratory5). The gene discussed is FKBP6; the disease is tuberculosis.