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Table 1 Summary of piRNA cluster identification methods and results from previous studies

From: proTRAC - a software for probabilistic piRNA cluster detection, visualization and analysis

study organism identified clusters criteria for detection of clusters
Aravin et al. 2006 [3] human/mouse 14/42 at least 4 piRNA loci per cluster, maximum distance between two piRNA loci 15 kb
Girard et al. 2006 [4] human/mouse/rat 186/123/157 at least 5 piRNA loci/5kb, at least 10 piRNA loci per cluster, only sequence reads that mapped 1-5 times to the genome were considered, sequence reads mapped to genome allowing up to 2 mismatches
Lau et al. 2006 [5] mouse/rat 94/100 at least 20 single-copy loci, at least 1 piRNA locus/kb
Lakshmi and Agrawal 2007 [7] human/mouse/rat 114/2710/189 at least n** single-copy loci/20kb, at least 2 piRNA loci/kb
Grivna et al. 2006 [12] mouse 35 *
Watanabe et al. 2006 [13] mouse 34 p < = 0.05, where p = (s/S)n-1×NCn S: genome size (bp), s: cluster size (bp), N: total number of sequence hits, n: number of hits in cluster
  1. *method not explained in more detail, ** based on hit density for each organism [7].