Phosphophoryns (PPs) are unique aspartic acid and phosphoserine‐rich proteins present in all species of dentin. Rat incisor odontoblast cDNA libraries contain messages encoding several acidic phosphorylated, serine‐rich proteins. At least two of these share a common C‐terminal domain coding region sequence. The polypeptide sequences in the N‐terminal direction immediately adjacent to the conserved C‐terminal domains of these two proteins (DMP2, DMP3) are distinctly different. In this domain, the DMP2 has extensive sequences of (DSS)n repeats with n as large as 24. DMP3 has fewer and shorter triplet sequences, n = 3, 4. The major rat incisor PPs (90–95 kDa) probably have the (DSS)n ≫ 3. We propose that the name phosphophoryn be reserved for the extracellular matrix proteins with these extended repeats. DMP1, although strongly acidic, does not fit this category. If the S residues are phosphorylated and n > 3. conformational energy minimization computations show the (DSS)n sequence to assume a unique extended structure with parallel arrays of carboxylate and phosphate groups which may function as Ca2+ ion interaction edges. The phosphorylation of recombinant DMP2 C‐terminal domain by various kinases has been examined. The repeat domains are not direct substrates for the CK2‐like kinases but the kinases act in concert, so that the phosphorylation is hierarchical, apparently controlled by the presence of specific interruptions between the triplet domains.