Isolated familial somatotropinoma (IFS) accounts for 18% of familial isolated pituitary adenoma (FIPA) cases. Recently, germline mutations of the aryl hydrocarbon receptor-interacting protein gene (AIP) have been found in families with pituitary adenoma predisposition, FIPA, and IFS. In this study, we investigate the AIP mutation status and perform a genome-wide scan to search for the modifier regions of acromegalic phenotypes in an IFS family of 31 aborigines from Borneo. Complete endocrine diagnosis and data could not be collected due to logistical and cultural reasons. AIP mutation screening was carried out by direct sequencing and the genome-wide scan was performed using 400 microsatellites. Non-parametric linkage analysis was performed to obtain the logarithm of odds (LOD) scores. A novel AIP frameshift mutation in exon 4 (c.500delC) (p.P167HfsX3) was identified in all members with acromegalic features, as well as in 15 members without acromegalic features, revealing incomplete penetrance of AIP. The data showed that patients with the same mutation may express acromegalic features of differing severity, suggesting the existence of modifier genes. The highest LOD score of 2.2 was obtained near D19S571 (19q13.41). We also found weak linkages on chromosomes 3q28, 8q12.1, and 21q22.13, with LOD scores of 1.1, 1.8, and 1.4 respectively. Our results show the first genome-wide scan that identifies novel modifier loci for acromegalic phenotypes in an IFS family. Identification of modifier loci may provide further insight into the disease mechanism and explain the clinical variability observed in its patients.