Complex sentence: Clinically, it is considered a premalignant (i.e. potentially malignant) condition, comprising one or more lesions containing cells that have already become biologically malignant ("cancerous") but have not yet crossed the basement membrane to invade the surrounding tissue.
Split into simpler sentences: Clinically, it is considered a premalignant (i.e. potentially malignant) condition. Each lesion contains cells that have already become biologically malignant ("cancerous") but have not yet crossed the basement membrane to invade the surrounding tissue.