Abstract Advanced stage melanoma is a mostly incurable, highly aggressive tumor whose incidence is rising. Metastatic dissemination of primary tumors involves the repeated crossing of tissues barriers, which requires dynamic modulation of tumor cell adhesion, presumably through mechanisms like those utilized by trafficking leukocytes. Activated leukocyte cell adhesion molecule (ALCAM/CD166) has been shown to play an important role in the metastatic process, including detachment from the primary tumor, adhesion to endothelium, and extravasation. ALCAM expression correlates with melanoma progression, and it has been identified as a marker of pluripotent stem cells. Furthermore, its expression in primary melanoma strongly correlates with tumor thickness and level of invasiveness. shRNA silencing of ALCAM in cells with high levels of ALCAM resulted in reduced motility and transmigration. In the current study, we tested the effect of CRISPR-mediated ALCAM knockout (KO) on migration and invasion of two human melanoma cell lines representing opposite phenotypic states of melanoma. MEL501 (proliferative type, SOX10/MITF-high and AP1/TEAD1-low) cells express low levels of ALCAM at baseline, while the Rosi cell line (invasive type, SOX10/MITF-low and AP1/TEAD1-high) expresses high baseline ALCAM levels. ALCAM KO promoted migration of MEL501 cells (p<0.000001) but inhibited migration of Rosi cells (p=0.005050) as compared to the parent cell lines. In addition, ALCAM KO promoted the invasion of MEL501 cells (p=0.019768) but had no impact on Rosi cell invasion. While ALCAM has been shown to regulate matrix metalloproteinase-2 (MMP-2) activation, zymograms revealed no difference in MMP-2 activity between MEL501 and Rosi ALCAM-KO cells and the parent cell lines. These data indicate that ALCAM may play a tumor-suppressive role in melanoma cells with low ALCAM expression and are consistent with a previous study which demonstrated that introduction of an amino-terminally truncated ALCAM in early stage melanoma cells promoted metastasis. Our data also provide in vitro evidence that gene therapy may reverse the migratory phenotype of melanoma cells with high ALCAM expression. Citation Format: Rafal Kaminski, Francesca P. Luongo, Konrad Dabrowski, Anna Bellizzi, Pietro Mancuso, M. Raza Zaidi, Johanna McMillan, Meha Patel, Tyrone Coleman, Oneida A. Arosarena. Baseline CD166 expression determines CRISPR-mediated ALCAM gene knockout effects on migratory phenotype in metastatic melanoma cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3287.