Given a context passage, generate a question from the passage such that its answer is shortest continous span from the passage.

Example Input: A common challenge in pathogen discovery by deep sequencing approaches is to recognize viral or subviral pathogens in samples of diseased tissue that share no significant homology with a known pathogen. Here we report a homology-independent approach for discovering viroids, a distinct class of free circular RNA subviral pathogens that encode no protein and are known to infect plants only. Our approach involves analyzing the sequences of the total small RNAs of the infected plants obtained by deep sequencing with a unique computational algorithm, progressive filtering of overlapping small RNAs (PFOR). Viroid infection triggers production of viroid-derived overlapping siRNAs that cover the entire genome with high densities. PFOR retains viroid-specific siRNAs for genome assembly by progressively eliminating nonoverlapping small RNAs and those that overlap but cannot be assembled into a direct repeat RNA, which is synthesized from circular or multimeric repeated-sequence templates during viroid replication. We show that viroids from the two known families are readily identified and their full-length sequences assembled by PFOR from small RNAs sequenced from infected plants. PFOR analysis of a grapevine library further identified a viroid-like circular RNA 375 nt long that shared no significant sequence homology with known molecules and encoded active hammerhead ribozymes in RNAs of both plus and minus polarities, which presumably self-cleave to release monomer from multimeric replicative intermediates. A potential application of the homology-independent approach for viroid discovery in plant and animal species where RNA replication triggers the biogenesis of siRNAs is discussed.
Example Output: Which are the smallest known subviral pathogens of plants?

Example Input: Hans Ernst von Kottwitz (1 September 1757 -- 13 May 1843) was a German Pietist and philanthropist.
Example Output: What was Hans Ernst von Kottwitz's nationality?

Example Input: Christianson syndrome (CS) is caused by mutations in SLC9A6 and is characterized by severe intellectual disability, absent speech, microcephaly, ataxia, seizures, and behavioral abnormalities. The clinical phenotypes of CS and Angelman syndrome (AS) are similar. Differentiation between CS and AS is important in terms of genetic counseling. We report on two children with CS and confirmed mutations in SLC9A6 focusing on neuroimaging findings and review the available literature. Cerebellar atrophy (CA) occurs in approximately 60% of the patients with CS and develops after the age of 12 months. Hyperintense signal of the cerebellar cortex (CbC) is less common, and may be diffuse, patchy, or involve only the inferior part of the cerebellum and is best seen on coronal fluid attenuation inversion recovery images. CA and CbC-hyperintensity are not neuroimaging features of AS. In a child with the phenotype of AS, CA and/or CbC-hyperintensity are rather specific for CS and should prioritize sequencing of SLC9A6.
Example Output:
Mutation of which gene is implicated in the Christianson syndrome?

