ABCA1 and Alzheimer disease: However, by individually analyzing the two contributions to total HDL-CEC, the efflux mediated by the aqueous diffusion (AD) process (AD HDL-CEC) and the efflux mediated by the ATP-binding cassette transporter A1 (ABCA1 HDL-CEC), we found a trend towards a reduction in AD HDL-CEC in FH patients presenting ATX compared to patients not presenting ATX (−8%; p = 0.053; Figure 1B).