That the sporadic and inherited forms of a particular cancer could both result from mutations in the same gene was first proposed by Knudson. He further proposed that these mutations act recessively at the cellular level, and that both copies of the gene must be lost for the cancer to develop. In sporadic cases both events occur somatically whereas in dominant familial cases susceptibility is inherited through a germline mutation and the cancer develops after a somatic change in the homologous allele. This model has since been substantiated in the case of retinoblastoma, Wilms tumour, acoustic neuroma and several other tumours, in which loss of heterozygosity was shown in tumour material compared to normal tissue from the same patient. The dominantly inherited disorder, familial adenomatous polyposis (FAP, also called familial polyposis coli), which gives rise to multiple adenomatous polyps in the colon that have a relatively high probability of progressing to a malignant adenocarcinoma, provides a basis for studying recessive genes in the far more common colorectal carcinomas using this approach. Following a clue as to the location of the FAP gene given by a case report of an individual with an interstitial deletion of chromosome 5q, who had FAP and multiple developmental abnormalities, we have examined sporadic colorectal adenocarcinomas for loss of alleles on chromosome 5. Using a highly polymorphic 'minisatellite' probe which maps to chromosome 5q we have shown that at least 20% of this highly heterogeneous set of tumours lose one of the alleles present in matched normal tissue. This parallels the assignment of the FAP gene to chromosome 5 (see accompanying paper) and suggests that becoming recessive for this gene may be a critical step in the progression of a relatively high proportion of colorectal cancers.

The purple acid phosphatases and uteroferrin belong to a diverse multifunctional class of binuclear ironcontaining proteins that includes haemerythrin and ribonucleotide reductase. In the pig, uteroferrin has been implicated in the delivery of iron to the foetus, but the role of the related human type 5 acid phosphatase that is principally found in resident tissue macrophages is not yet clear. To define further the function of this metalloenzyme, we have isolated and sequenced a cDNA clone for type 5 acid phosphatase and investigated expression of its gene in human tissues. The phosphatase clone contains an open reading frame of 975 bp and encodes a protein of 325 amino acids, including a signal peptide of 19 residues and two potential sites for N-glycosylation. The type 5 acid phosphatase gene mapped to the short arm of human chromosome 19 and was found to have a restriction fragment length polymorphism on digestion with XbaI.Expression of phosphatase mRNA was restricted to mononuclear phagocytes and the enzyme was induced > 20-fold on transformation of normal human monocytes to macrophages by culture in serum-supplemented medium. Type 5 acid phosphatase thus represents a tightly regulated system for the study of molecular events in the differentiation programme of the normal macrophage.Type 5, tartrate-resistant, purple acid phosphatase is a basic, iron-binding protein with high activity towards phosphoproteins, ATP, and 4-nitrophenyl phosphate but with little action on aliphatic phosphates [I, 21. The isoenzyme is found in human alveolar macrophages [3] and osteoclasts [4] and is pathologically increased in Gaucher's cells [5] and in the hairy cells of leukaemic reticuloendotheliosis [6], now known as hairy cell leukaemia. It is thus expressed principally in cells of the human mononuclear macrophage system.Recently the protein has been purified from human and bovine spleen, where it is associated with a particulate fraction [7, 81. lmmunochemical and enzymatic studies [9, 101 as well as protein sequence analysis [I 11 have shown that the splenic enzyme is homologous to uteroferrin, a purple iron-binding protein abundant in the uterine secretions of pregnant sows. Physiological studies in vivo have implicated uteroferrin in the delivery of maternal iron to the developing piglet [12, 131 but the possibility of a similar function for the human tartrateresistant acid phosphatase has not been investigated. However, ultrastructural studies [14] indicate association of the bovine protein with ferritin and haemosiderin aggregates in splenic macrophages, suggesting that it may participate in the Correspondence to

We used somatic-cell hybrids, containing as their only human genetic contribution part or all of chromosome 17, as donors for chromosome-mediated gene transfer. A total of 54 independent transfectant clones were isolated and analyzed by use of probes or isoenzymes for >20 loci located on chromosome 17. By combining the data from this chromosomemediated gene transfer transfectant panel, conventional somatic-cell hybrids containing well-defined breaks on chromosome 17, and in situ hybridization, we propose the following order for these loci: pter- (i) although substantial lengths of DNA may be transferred intact (8, 9), interstitial deletions do frequently occur; (ii) multiple fragments of transfected chromatin can be found in the same clone; and (iii) there is also selection for centromeric sequences (8,9).In addition, during the course of this study we obtained some transfectants that contained only the small regions of chromosome 17 particularly relevant to the study of APL and von Recklinghausen neurofibromatosis. MATERIALS AND METHODSCulture Conditions. The cells were cultured either in RPMI 1640 or in Dulbecco's minimal essential medium supplemented with 10% fetal bovine serum. PCTBA1.8, PJT2/A1, and all CMGT transfectants, PLT, KLT, and TLT were maintained in HAT medium (100 ,uM hypoxanthine/10 ,M methotrexate/10 ,M thymidine), which selects for thymidine kinase. The hybrids GPT17.3.2K41 and TRID62 were maintained in MX medium [mycophenolic acid (25 ,ug/ml)/xanthine (250 ,g/ml)]. Back selection of CMGT transfectants with 5-bromo-2-deoxyuridine was done by the addition of 5-bromo-2-deoxyuridine (50 ,g/ml) to the medium, thus selecting for those CMGT transfectants that have lost the TKI locus. This medium was replenished every 3 days for 2-3 weeks. Resulting colonies were maintained in RPMI 1640/ 10% fetal bovine serum and were denoted by the letter B after the name of the original transfectant.Cytogenetic Analysis. Chromosomes from human lymphocytes, hybrids, and CMGT transfectants were all prepared as described (10). Additional

The human c-erbA1 gene has been previously mapped to chromosome 17. We have now mapped c-erbA2 to the short arm of chromosome 3, using a human genomic probe in Southern analysis of DNA from a panel of human/mouse somatic cell hybrids. In situ hybridization using the same probe on metaphase chromosomes has enabled fine chromosome mapping of c-erbA2 to the chromosome region 3p21-pter.

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