Abstract
121 mice homozygous for the gene seb (inherited seborrhoeic dermatitis) and their 142 unaffected heterozygous littermates were kept for their natural lifespan. Heterozygotes showed 84.1% total tumour incidence in males and 95.9% in females. The most common neoplasms were lymphomas, osteomas, lung tumours and neoplasms of the female genital tract. Homozygotes showed a tumour incidence of 36.1 % in males and 45.0% in females. The reduction in incidence included all types of neoplasms except epithelial tumours of the skin: skin tumours were detected in 11 homozygous but only in one heterozygous animal. Life expectancy was not affected significantly by genotype. Homozygous mice showed rough and greasy fur and became alopecic with age. Energy intake was increased but growth and depository fat was reduced compared with heterozygous mice. Higher heat loss may incompletely be compensated by higher metabolic rate and thus 'dietary restriction' results in decreased tumour rates. As females show small gonads and a higher increase in food consumption hormonal factors may also be involved.