Some teachings on Buddha Nature by His Holiness the Dalai Lama:
"Within the Mahayana, the term buddha nature has different shades of meaning. In the Mind-only school, buddha nature refers to our fundamental uncontaminated mind that, when untapped, is said to be our "naturally abiding' buddha nature, and when awakened is said to be our 'transformed' buddha nature. This naturally abiding buddha nature is also known as natural nirava, or natural liberation, for it exists naturally in all of us. It is because of the presence of this natural nirvana that the pollutants obscuring its expression are said to be separable from the essential nature of the mind, making enlightenment possible. In the Middle Way School, buddha nature is defined differently: it is defined in terms of emptiness, specifically, the mind's emptiness of intrinsic existence. This is also called the Clear- light nature of the mind."
See more in The Essence of the Heart Sutra - Heart of the Wisdom Teachings by The Dalai Lama.
And from Beyond Dogma - His Holiness the Dalai Lama
Q: How did illusion begin?
His Holiness the Dalai Lama: As the continuum of the mind has no beginning, ignorance does not have one either. If it did, we would have to discover from within a state of consciousness that predates ignorance and is different from it in an enlightened mind, -- a cause resulting in ignorance. This makes no sense.