This domain represents a conserved region related to cystatins. Eight copies of which are found within the plant phloem filament protein PP1. This is one of the constituents of the proteinaceous filaments found in the sieve elements of Cucurbita phloem [1].

1: Molecular characterization of a phloem-specific gene encoding the filament protein, phloem protein 1 (PP1), from Cucurbita maxima. Clark AM, Jacobsen KR, Bostwick DE, Dannenhoffer JM, Skaggs MI, Thompson GA; Plant J 1997;12:49-61. PMID:9263452