the factors proposed to influence biological aging fall into two main categories, "programmed" and "damage-related". programmed factors follow a biological timetable, perhaps one that might be a continuation of the one that regulates childhood growth and development. this regulation would depend on changes in gene expression that affect the systems responsible for maintenance, repair and defense responses. damage-related factors include internal and environmental assaults to living organisms that induce cumulative damage at various levels. a third, novel, concept is that aging is mediated by vicious cycles.