Genetic regulatory networks controlling early PLM and ALM development. Direct relationships, illustrated by continuous lines, are defined by 3 criteria: (1) target gene expression is affected by perturbation of activator; (2) target gene and activator are coexpressed; (3) target gene promoter/enhancer sequences contain binding sites for activators, or length of time between perturbation of activator and effect on target gene is probably insufficient to allow for synthesis of intermediates. Where criteria 1 and 2 are met but 3 is unknown, the relationship is described as indirect and depicted by a dashed line. Lmo2 does not bind DNA, but is known to be an obligate member of a multiprotein complex containing Scl (represented here by the “and” function).^15,37  PLM (Erythroid): A gata1 enhancer contains binding sites for the multiprotein complex containing Scl and Lmo2.⁴⁹  Our data show that initiation of gata1 expression is Scl-dependent. Initially, runx1 expression is dependent on Scl, whereas hhex and dra are Scl-independent. After 7 somites (12.5 hpf), hhex and dra become dependent on Scl, whereas runx1 expression gradually becomes Scl-independent. ALM (Myeloid): Pu.1 expression is initially dependent on Scl, whereas hhex and dra are independent. After 7 somites (12.5 hpf), hhex and dra become Scl-dependent. Unknown activators of Scl-independent genes are depicted as genes X, Y, and Z. It is possible that X and Y represent the same gene due to similarities in timing of involvement.