*2.1. Tissue Localization of ABCA7*

ABCA7 was first cloned in 2000 by Kaminski and colleagues from human macrophages [11]. Distribution/expression of the ABCA7 mRNA in human tissues demonstrated that it is highly expressed in myelo-lymphatic tissues (peripheral leucocytes, thymus, spleen, bone marrow, fetal tissues) [11]. Preferential and high expression of *Abca7* mRNA in lymphomyeloid tissues was immediately confirmed in mice and rats [13,14], strongly suggesting a key role of this transporter in hematopoietic cell lineages. More recently, analysis based upon ImmGen Deep RNA-seq data showed that *Abca7* is one of the most highly expressed ABC transporter in a purified population of mouse immune cells like follicular B cells, NK cells, peritoneal macrophages, thus highlighting a role for ABCA7 in immunity [15]. In addition, it appears that ABCA7 expression is dependent of the differentiation state of the cells since higher expressions were measured in differentiated macrophages comparing to monocytes [11].

Interestingly, no ABCA7 mRNA signal was initially detected in total human brain samples but this result was later refuted by several groups demonstrating mRNA and protein expression of ABCA7 in human and murine samples of the different cell types present in the brain [16–23]. Therefore, it is now largely demonstrated that ABCA7 is expressed in neurons, astrocytes, microglia, endothelial cells of the blood–brain barrier (BBB), brain pericytes, not only in mice but also in humans ([16–24]—http://www.brainrnaseq.org/, http:// www.celltypes.org/ and https://www.proteinatlas.org/ENSG00000064687-ABCA7/brain, accessed on 31 December 2020).
