Citrus exocortis is a disease of citrus plants, caused by the Citrus exocortis viroid (CEVd). The mode of natural inoculation of the viroid is unknown. Symptomless trees also represent a greater danger in terms of spread of the viroid. The Pospiviroidae are a family of viroids, including the first viroid to be discovered, PSTVd. It is however thought that RY motifs are essential for establishing a viroid infection. This viroid is now called Potato spindle tuber viroid, abbreviated PSTVd. Avocado sunblotch viroid (ASBV) is a disease affecting avocado trees. First, changes to the viroid genome can dramatically alter its virulence. The human pathogen hepatitis D virus is a subviral agent similar to a viroid. Secondly, siRNAs corresponding to sequences from viroid genomes have been isolated from infected plants. Heat does not kill the viroid. Viroid RNA does not code for any protein. The Potato spindle tuber viroid ("PSTVd") was the first viroid to be identified. This viroid has 64% sequence homology with the cadang-cadang viroid. Infected but symptomless trees have a higher concentration of viroid particles than those showing symptoms.