  It has been argued, against gene skepticism, that biologists have a coherent, precise, and uniform way to conceive of genes at the molecular level. The analysis underlying this argument begins by distinguishing between two different ways contemporary geneticists think about genes. Classical geneticists often conceived of genes as the functional units in chromosomes, differences in which cause differences in phenotypes. Today, in contexts where genes are identified by way of observed phenotypic differences, geneticists still conceive of genes in this classical way, as the functional units in DNA whose differences are causing the observed differences in phenotypes. This way of conceiving of genes is called the classical gene concept (Waters 1994). But contemporary geneticists also think about genes in a different way by invoking a molecular-level concept. The molecular gene concept stems from the idea that genes are units in DNA that function to determine linear sequences in molecules synthesized via DNA expression. According to this analysis, both concepts are at work in contemporary geneticists.  Moss 2003 also distinguishes between two contemporary gene concepts, which he calls “genes-P (preformationist)” and “genes-D (developmental)”. He argues that conflation of these concepts leads to erroneous thinking in genetics.