ABL1 and chronic myelogenous leukemia, BCR-ABL1 positive: In addition to population heterogeneity, changes in functional relationships between response variables and explanatory variables, particularly with time, are ubiquitous in longitudinal studies: HIV-1 viral load [9, 10], hepatitis B/C viral load [11, 12] and BCR-ABL expression levels in chronic myeloid leukemia [13, 14].