Although at the beginning of an HIV-1 infection, the viruses have identical genomes, these can diversify over time due to the appearance of spontaneous mutations [
71,
131]. However, clusters of identical viral sequences originating from clonally expanded infected cells are often observed in patients undergoing long-term antiretroviral therapy [
132,
133]. In patients treated with an efficient ART, plasma viremia can be reduced by more than ten thousand times, while the level of intracellular viral DNA decreases much less, on average, by only fifteen times [
134]. This phenomenon has been related to a clonal expansion of infected cells hosting latently integrated provirus [
134]. Upon suspension of ART, there is a rapid increase in plasma viremia. This indicates the presence of some cells in the latent reservoir capable of re-expanding and activating HIV-1 transcription [
135,
136,
137]. Evidence that the provirus often integrates into a cancer-associated gene has suggested that this type of integration potentially can confer a proliferative advantage to clonal populations of infected cells [
11]. The viral promoter LTR could grant the proliferative advantage, activating oncogenic gene expression [
11]. However, other authors have demonstrated that integrating HIV-1 in cancer-related genes is a minor factor in the clonal expansion of infected CD4+ T cells [
138]. Furthermore, HIV-1 proviruses integrated into or close to cancer-related genes are often defective and unable to determine the increase in viremia upon interruption of ART [
138]. There is another category of genes, the
KRAB-ZNF genes (
KRAB = Krüppel associated box;
ZNF = Zinc finger), where the insertion of HIV-1 proviruses is observed, especially in patients treated with long-term ART [
12,
83,
84,
85,
139,
140,
141]. KRAB-ZNF proteins produced by these genes are the most abundant family of epigenetic repressors found only in tetrapod vertebrates. Upon binding to DNA, KRAB-ZNF proteins trigger transcriptional repression via interaction with TRIM28, which acts as a scaffold for heterochromatin protein 1 (HP1), H3K9me3-specific histone methyltransferase 1 (SETDB1), and the NuRD histone deacetylase-containing complex. Together, these proteins silence transcription by triggering H3K9 trimethylation and heterochromatin formation [
142,
143]. Huang et al. analyzed integration sites of near full-length HIV-1 genomes from individuals on long-term ART. They observed that, in clonally expanded cells of these individuals, proviruses were preferentially integrated within
KRAB-ZNF genes. Unlike oncogenic genes,
KRAB-ZNF genes do not promote cell proliferation, so the clonal expansion of CD4+ T-cells containing provirus integrations in
KRAB-ZNF genes can be explained by the lack of production of the virus that is latently integrated, and thus the lack of cytolytic effect.
KRAB-ZNF genes are associated with heterochromatin in memory CD4+ T cells, so they are under-expressed. [
85]. Other studies indicate that HIV-1 proviruses integrated into
KRAB-ZNF genes are intact and not defective and are only partially refractory to induction, suggesting that these genes might represent the region of host DNA where the provirus can integrate latently but can also be reactivated, for example, by ART withdrawal [
12,
144].