DLK1 and type 2 diabetes mellitus: DLK1 was previously noted to be enriched in subpopulations of β-cells (Baron et al., 2016; Li et al., 2016; Lawlor et al., 2017; Mawla and Huising, 2019; Xin et al., 2016); however, its heterogeneity in transcript and protein abundance in ND versus T2D human islet β-cells had not been described until now.