Lama Zopa Rinpoche:
Due to holding onto the projection of a false I, we are continuously, all the time, nonstop, creating the root of saṃsāra. In fact, the root of suffering is nothing other than this. Instead of seeing the I as merely labeled by mind, we see it as existing from its own side. We have been believing in this I, which exists as merely labeled by the mind but which appears as not merely labeled by mind, not just from this morning or from this birth but from before this—from beginningless rebirths.
Each time we believe in this we are creating the ignorance that is the root of all negative emotional thoughts, all the delusions that produce karma. All the suffering of pain, all the suffering of change (which includes all the temporary saṃsāric pleasure, which is only in the nature of suffering) as well as the foundation of these two—the pervasive compounding suffering from which the suffering of pain and change arise like bubbles on water, or waves on the ocean—come from this mistaken view.
All the three realms of desire, form and formlessness are produced and come about because we are controlled by karma and delusion. Our current aggregates are also the product of the contaminated seed of delusion. As long as it is extant, this imprint, or seed, compounds the sufferings of this life: those of the body, such as physical problems, and the mind, such as loneliness and depression. Because delusion arises from the seed of delusion already on the mindstream, it also compounds future life sufferings, and since we don’t meditate in our daily life, we have no protection from these seeds ripening. When we encounter objects, whether ugly or beautiful, the delusions of anger, attachment and ignorance are generated. A motivated karma then leaves an imprint, or seed, on the mental continuum or, according to the Prāsaṅgika School, on the mere I that produces (or impels) the future rebirth.
Every time we let our mind hold onto this I that appears to us as not merely labeled by mind and then hold onto it with the belief that it is true, we are creating more ignorance. Ignorance is not inherently existent; ignorance is not independent. We create it twenty-four hours a day.
 False I (rdzun pa’i nga).
 The use of “mere” (tsam, mātra), as in “merely labeled,” can also be translated as “simply,” or “only.” As Tsongkhapa notes, as a qualifier it bears great significance: “the term ‘only’ eliminates anything that is not a subjective convention; it does not at all eliminate that the object posited is established by valid cognition.” See Hopkins, Final Exposition, 128–29.
 Karma (or “formative activity”) refers here to that which constitutes the second link of the twelve links of dependent origination. Explains Sopa, “Formative activity refers to the actions that arise because of ignorance, which become the causes and conditions that eventually bring all future results. In short, this refers to karma. Karma is action of body, speech, and mind, and there are two types: nonvirtuous or demeritorious karma, which produces unhappy rebirths, and virtuous karma, which produces happy rebirths….For all samsaric karma, the underlying cause is ignorance.” Steps, Volume 2, 327. Considered more generally, we can consider karma to mean “action.” As Quarcoo notes, “In Tibetan the word for ‘action’ and ‘karma’ is the same (las).” See Tsongkhapa, Middle Length Lam-Rim, 78, footnote 98. See also the Dalai Lama, Harvard, 59–60 for a concise general account.
 His Holiness the Dalai Lama gives a succinct account of the same point in his commentary to the Paṇchen Lama’s poem entitled “Wishes for Release from the Perilous Straits of the Intermediate State, Hero Releasing from Fright.” See Advice on Dying, 101. See also a concise account of the three sufferings experienced by sentient beings together with the need to renounce them in Dhargyey, Tibetan Tradition, 21–23.
 The reference is to our five aggregates (phung po, skandha) of body and mind (the five appropriated, or taken up, aggregates, nyer len gyi), the body being the form aggregate and the mind consisting of the four mental aggregates: feeling, cognition, compositional factors and consciousness. These are often called the personal aggregates because the person is labeled in dependence upon them. For detail, see Tegchok, Insight, 59–64.
 In the current context (there are multiple classifications) karma can be considered under two sub-headings: projecting or throwing karma (’phen byed kyi las), translated here by Lama Zopa Rinpoche as “motivated karma”—and completing, or actualizing karma (rdzogs byed kyi las): “Projecting karma is the strong karma that projects or throws one into a particular type of rebirth. This is like planting a seed. It is the primary cause of the future rebirth—the cause of being born in a particular realm of existence. The second type is called completing karma or actualizing karma. This is what actualizes a projecting karma.” Geshe Sopa, Steps, Volume 2, 295. Geshe Gyatso illustrates: “The projecting karma from an act of killing might for instance project or throw us into a lower state of existence, such as being born a dog. Then completing karma deriving from a previous positive action might ripen during that life as a dog such that we are sheltered by kind people who provide us with excellent food and other comforts.” Lobsang Gyatso, The Harmony of Emptiness and Dependent-Arising (Dharamsala: Library of Tibetan Works and Archives, 2006), 23. For a famous example of the relation between throwing and completing karma (in regards to a person reborn as a man with an extremely ugly visage yet with an astoundingly beautiful singing voice), see Tsongkhapa, Middle Length Lam-Rim, 72 and 93.
In regards to the complex and vital question as to what carries the karmic imprint:
When we speak of imprints, we are speaking of the mental propensities created by the karma collected in the past that is stored until conditions assemble for those propensities to be released. It is imprints that connect the committing of acts in the past o the reaping of their fruits at some future time. This is why the repository of the imprints must have a stable continuum. To address this, some past Buddhist masters posited a foundational consciousness (ālayavijñana) as the basis of imprints. Others believing that the essence of the person can be found when sought critically, consider the consciousness they identified as the essence of the person as the basis of the imprints. Both of these explanations creative a basis for explaining how karmic imprints are carried from life to life, but they also lead to other philosophical problems. The way Nagarjuna’s Madhyamaka school approaches it is different.
When Chandrakriti interpreted Nagarjuna’s unique standpoints, he drew a distinction between a temporary basis os the imprints and a more enduring basis. he said that the immediate consciousness contemporaneous with the karmic act was the temporary basis of the imprint,, while the enduring basis of the imprints is the I or the “person” that is constructed on the basis of the continuity of the person’s consciousness. Neither of these bases can be found when the true referents of their terms are sought through critical enquiry. They are mere conceptual constructs, albeit conventionalities not negated by some other conventionally valid cognition.
See His Holiness, The The Middle Way: Faith Grounded in Reason. Translated by Thupten Jinpa (Boston: Wisdom Publications, 2009), 34-5.