Molecular SIBLINGs in Health and Disease  The SIBLING gene
family  A family of secreted proteins, termed SIBLINGs
(for SmallIntegrin-Binding LIgand, N-linked Glycoproteins),
contains the integrin-binding tripeptide, Arg-Gly-Asp (RGD), as
well as several conserved phosphorylation and N-glycosylation
sites [1]. All of the genes cluster on human chromosome 4 and
are normally restricted in expression to skeletal tissue. There
are currently six known members of the SIBLING family: bone
sialoprotein (BSP), osteopontin (OPN), dentin matrix protein
(DMP1), dentin sialophosphoprotein (DSPP), matrix extracellular
phosphoglycoprotein (MEPE), and enamelin (ENAM) Fig. 1 [2].  
Figure 1. The SIBLING family cluster and structure. (A) Genes
encoding bone-specific sialoproteins I and II (OPN and BSP),
dentin-specific proteins (DMP1 and DSPP), enamelin-specific
protein (ENAM) and a skeletal protein (MEPE) have been localized
to the same region of chromosome 4q21 – 25. (B) Characteristics
of this family include 1) an integrin-