33The prefrontal cortex (PFC) and its reciprocal connections with the mediodorsal 34 thalamus (MD) are crucial for cognitive flexibility and working memory 1-4 and are 35 thought to be altered in several disorders such as autism spectrum disorder 5, 6 36 and schizophrenia 6-9 . While developmental mechanisms governing regional 37 patterning of the rodent cerebral cortex have been characterized 10-15 , the 38 mechanisms underlying the development of PFC-MD connectivity and the lateral 39 expansion of PFC with distinct granular layer 4 in anthropoid primates 16-23 have 40 not been elucidated. Here we report increased concentration of retinoic acid (RA), 41 a signaling molecule involved in brain development and function 24,25 in the 42 prospective PFC areas of human and macaque, compared to mouse, during mid-43 fetal development, a crucial period for cortical circuit assembly. In addition, we 44 observed the lateral expansion of RA synthesizing enzyme, ALDH1A3, 45 expression in mid-fetal macaque and human frontal cortex, compared to mouse. 46 Furthermore, we found that enrichment of RA signaling is restricted to the 47 prospective PFC by CYP26B1, a gene encoding an RA-catabolizing enzyme 48 upregulated in the mid-fetal motor cortex. Gene deletion in mice revealed that RA 49 signaling through anteriorly upregulated RA receptors, Rxrg and Rarb, and 50 Cyp26b1-dependent catabolism is required for the proper molecular patterning 51 of PFC and motor areas, the expression of the layer 4 marker RORB, intra-PFC 52 synaptogenesis, and the development of reciprocal PFC-MD connectivity. 53