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Simple Summary: This study aimed to determine the gene expression profiles associated with chemotherapeutic responses in conventional osteosarcomas (COS) within South Africa. We observed a significant downregulation in the ATP binding cassette subfamily C members (ABCC3 and ABCB1-p-glycoprotein), excision repair cross-complimenting group 1 (ERCC 1), replication factor C subunit 1 (RFC1), and tumour protein 53 (p53) genes in the COS tumours compared to the healthy donors. Furthermore, an upregulated ERCC1 gene expression level predicted a poor chemotherapeutic response. Additionally, the predictors of COS chemotherapeutic response comprised age, chondroblastic and osteoblastic histological subtypes, and ABCC3, ERCC1, and RFC1 gene expression. Background: We determined the predictive gene expression profiles associated with chemo-response in conventional osteosarcomas (COS) within South Africa. Materials and methods: In 28 patients, we performed an RNA extraction, cDNA synthesis, and quantitative analysis using the RT-PCR 2−∆∆CT method to determine the fold change in gene expression alongside GAPDH (housekeeping gene). Results: We observed a significant downregulation in the mRNA expression profiles of ABCB1-p-glycoprotein (p = 0.0007), ABCC3 (p = 0.002), ERCC1 (p = 0.007), p-53 (p = 0.007), and RFC1 (p = 0.003) in the COS patients compared to the healthy donors. Furthermore, ABCB1-p-glycoprotein (p = 0.008) and ABCC3 (p = 0.020) exhibited a significant downregulation in the COS tumour tissues when compared to the healthy donors. In our univariate logistic regression, the predictors of chemotherapeutic response comprised ERCC1 [restricted cubic spline (RCS) knot: OR −0.27; CI −0.504 to −0.032; p = 0.036]; osteoblastic subtype [OR −0.36; CI −0.652 to −0.092; p = 0.026); fibroblastic subtype [OR 0.91; CI 0.569 to 1.248; p < 0.001]; and mixed subtype [OR 0.53; CI 0.232 to 0.032; p = 0.032]. In our multivariable logistic regression, the significant predictors of chemotherapeutic response comprised age [RCS knot: OR −2.5; CI −3.616 to −1.378; p = 0.022]; ABCC3 [RCS knot: OR 0.67; CI 0.407 to 0.936, p = 0.016]; ERCC1 [RCS knot: OR 0.57; CI 0.235 to 0.901; p = 0.044]; RFC1 [RCS knot: OR −1.04; CI −1.592 to −0.487; p = 0.035]; chondroblastic subtype [OR −0.83; CI −1.106 to −0.520; p = 0.012]; and osteoblastic subtype [OR −1.28; CI −1.664 to −0.901; p = 0.007]. Conclusions: In this South African cohort, we observed the unique gene expression profiles of osteosarcoma tumourigenesis and chemotherapeutic responses. These may serve as prognostication and therapeutic targets. Larger-scale research is needed on the African continent. [ABSTRACT FROM AUTHOR]