The presence of ectopic thymus tissue. Normally, cells of the ventral bud of the third pharyngeal pouch detach and migrate in the eighth gestational week caudally and medially towards the location of the mature thyroid. They migrate further retrosternally into the superior mediastinum. There are two main ways ectopic thymus tissue can develop. Either cells detach along the descensus path and proliferate, thereby forming accessory thymus tissue, or the entire gland fails to descend. The phenotype is Human Phenotype Ontology term HP_0010517 (Ectopic thymus tissue). Also known as: Abnormal thymus position.