SUMMARY We report the construction and analysis of 4,836 heterozygous diploid deletion mutants covering 98.4% of the fission yeast genome. This resource provides a powerful tool for biotechnological and eukaryotic cell biology research. Comprehensive gene dispensability comparisons with budding yeast, the first time such studies have been possible between two eukaryotes, revealed that 83% of single copy orthologues in the two yeasts had conserved dispensability. Gene dispensability differed for certain pathways between the two yeasts, including mitochondrial translation and cell cycle checkpoint control. We show that fission yeast has more essential genes than budding yeast and that essential genes are more likely than non-essential genes to be single copy, broadly conserved and to contain introns. Growth fitness analyses determined sets of haploinsufficient and haploproficient genes for fission yeast, and comparisons with budding yeast identified specific ribosomal proteins and RNA polymerase subunits, which may act more generally to regulate eukaryotic cell growth.

Although mucins have been studied at the biochemical and biophysical level for some time, attempts to define their structures in detail were only partially successful because of their size and complexity. The advent of monoclonal antibodies reactive with these molecules introduced a new approach to structural studies by defining antigenic epitopes, by allowing purification of the mucin molecules by affinity chromatography, and by providing a means to clone genes coding for the core proteins. By their profile of reactivity with the normal and cancer-associated mucin in a particular tissue, the antibodies also defined a difference in the mucin derived from the two sources. It is now clear that this difference lies in the carbohydrate side chains, as the core proteins are identical. Because the mucins are tumor-associated antigens and the cancer mucins can express epitopes that are relatively tumor specific, this family of molecules is now being intensively studied. There is also considerable interest in elucidating the normal function of the mucin and in determining whether, through an altered structure, this function is subverted in malignancy. In the next few years we should expect that the structure of other mucins will be defined in the same detail as the product of the MUC1 gene. We should also expect to see the continued application of mucin-reactive antibodies in the clinic and the investigation of mucins as agents for immunotherapy of some cancers. As to the function(s) of these molecules, perhaps we will learn enough in the future to make a critical reappraisal of the name.

The polymorphic epithelial mucin (PEM) appears to be the target molecule for many monoclonal antibodies (MAbs) which react with tumour-associated and epithelium-specific antigens. PEM contains a large domain made up of 20 amino-acid tandem repeats which are highly immunodominant as many of the antibodies reactive with this molecule recognize epitopes within this area. Using overlapping peptide octamers, we have precisely mapped the epitopes of 4 MAbs reactive with the tandem repeats including one, SM-3, which shows enhanced tumour specificity. We report that the core of the SM-3 epitope corresponds to the continuous amino acid sequence Pro-Asp-Thr-Arg-Pro. We also show that the epitopes recognized by 3 other antibodies, which show reactivity with normal and malignant tissues, map to within this area of the tandem repeat. However, none of these epitopes contain the proline found at the amino end of the SM-3 determinant. These results are consistent with the idea that, in the cancer-associated mucin, premature termination of the carbohydrate side-chains results in the exposure of the SM-3 epitope.

Human mammary epithelial cells secrete and express on their cell surfaces complex mucin glycoproteins (Mr > 250,000) that are developmentally regulated, tumorassociated, and highly immunogenic. Studies using monoclonal antibodies directed to these glycoproteins suggest that their molecular structures can vary with differentiation stages in the normal gland and in malignancy. To analyze the molecular nature of these glycoproteins, milk mucin was affinity-purified and deglycosylated with hydrogen fluoride, yielding bands at 68 and 72 kDa on silver-stained gels. Polyclonal and monoclonal antibodies to the stripped core protein were developed and used to screen a Xgtll expression library of cDNA made from mRNA of the mammary tumor cell line MCF-7. Seven crossreacting clones were isolated, with inserts 0.1-1.8 kilobases long. RNA blot analysis, using as a probe the 1.8-kilobase insert subcloned in plasmid pUC8 (pMUC10), revealed transcripts of 4.7 and 6.4 kilobases in MCF-7 and T47D mammary tumor cells, whereas normal mammary epithelial cells from pooled milks have additional transcripts. The expression of mRNA correlates with antigen expression as determined by binding of two previously characterized anti-mucin monoclonal antibodies (HMFG-1 and HMFG-2) to seven cell lines. Restriction enzyme analysis detected a restriction fragment length polymorphism when human genomic DNA was digested with EcoRI or Hinfl.Normal and malignant human mammary epithelial cells express high molecular weight glycoproteins that are extensively glycosylated and very antigenic. As a result, many of the monoclonal antibodies (mAbs) selected for reactivity with human breast cancers are found to react with these components (1-7). These antibodies generally also react with molecules that are produced in abundance by the fully differentiated human mammary tissue and that are found in the milk fat globule and in milk. However, the level of expression of a particular antigenic determinant may be different in the glycoproteins produced by the normal differentiated cell and in the immunogenically related molecules produced by breast cancers (1,(6)(7)(8)(9). This means that some antibodies can show a certain specificity for reacting with the tumor glycoproteins, although this is quantitative rather than qualitative. The molecules expressing the epitopes recognized by these antibodies have been difficult to analyze, both because they are large (>250 kDa) and heavily glycosylated and because of their complex pattern of expression. Use of two mAbs, HMFG-1 and HMFG-2 (originally called 1.10.F3 and 14.A.3, respectively; ref. 10), has shown that the reactive component in human milk appears to be >400 kDa, while the molecules carrying the antigenic determinants in sera and tumors are smaller [but generally still >200 kDa (1)] and exhibit a genetic polymorphism (11).The large glycoprotein produced by the differentiated mammary epithelial cells and found in human milk or in the milk fat globule has been purified and shown to have features characteris...

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