Sequence similarity between netrin UNC-6 and C345C complement protein family members, and hence the existence of the UNC-6 module, was first reported in [1]. Subsequently, many additional members of the family were identified on the basis of sequence similarity between the C-terminal domains of netrins, complement proteins C3, C4, C5, secreted frizzled-related proteins, and type I pro-collagen C-proteinase enhancer proteins (PCOLCEs), which are homologous with the N-terminal domains of tissue inhibitors of metalloproteinases (TIMPs). The TIMPs are classified as a separate family in Pfam (Pfam:PF00965) [2]. This expanded domain family has been named as the NTR module [2].

1: UNC-6, a laminin-related protein, guides cell and pioneer axon migrations in C. elegans. Ishii N, Wadsworth WG, Stern BD, Culotti JG, Hedgecock EM; Neuron 1992;9:873-881. PMID:1329863

2: The NTR module: domains of netrins, secreted frizzled related proteins, and type I procollagen C-proteinase enhancer protein are homologous with tissue inhibitors of metalloproteases. Banyai L, Patthy L; Protein Sci 1999;8:1636-1642. PMID:10452607