SPP1 and cancer: A number of the differentially expressed genes is associated with tissue/organ/system development, such as gastrulation brain homeobox 2 (GBX2) and early growth response 2 (EGR2) [50,  51], or cancer/apoptosis, such as protein kinase C delta (PRKCD), secreted phosphoprotein 1 (SPP1/OPN), prostaglandin-endoperoxide synthase 2 (Ptgs2; also named cyclo-oxygenase 2, COX-2) and growth arrest and DNA-damage-inducible 45 gamma (GADD45g) [52-55].