Every solid tumor, whether it is a tumor growing in a cancer patient, or a transplantable tumor grown as a rodent model of cancer, contains populations of both P and Q cells. 27 The number of P and Q cells in a tumor is highly variable from one patient to the next, and represents a significant issue in the clinical management of cancer. For example, radiation and most chemotherapeutics kill P cells more effectively than Q cells, and clonogenic Q cells can often survive following the completion of radiotherapy and chemotherapy. 28 Therefore, knowledge of the GF and PS of a tumor can provide useful information to oncologists in determining an appropriate strategy for treating cancer patients by either chemotherapy or radiation therapy. 29 , 30 Tumors having a high GF and PS typically respond better to hyperfractionated radiation therapy versus conventional radiation therapy. 31 Similarly, tumors having a high GF and PS will respond better to cell cycle specific agents such as Ara-C and gemcitabine, whereas tumors having a lower proliferative status will respond better to non-cell cycle specific agents such as cisplatin and BCNU. 29 Finally, GF and PS can be used to identify patients who will receive the maximum benefit from newer cancer treatment strategies (., Polo-like kinase inhibitors, cyclin dependent kinase and Chk-1 inhibitors) since these drugs target various proteins that are expressed in cycling cells but which are not expressed in quiescent tumor cells. 32 , 33 ( Table 1 )
For those sequences which have a structure in the Protein DataBank , we use the mapping between UniProt , PDB and Pfam coordinate systems from the PDBe group, to allow us to map Pfam domains onto UniProt sequences and three-dimensional protein structures. The table below shows the structures on which the Cyt-b5 domain has been found. There are 99 instances of this domain found in the PDB. Note that there may be multiple copies of the domain in a single PDB structure, since many structures contain multiple copies of the same protein seqence.