This product listing is for 3 Vials, 2000mcg each, total: 6000mcg
Modified GRF (1-29) (“Mod GRF 1-29)”), also called tetrasubstituted GRF or tetrasubstituted GHRH, is a twenty-nine amino acid synthetic analog of the endogenous hypothalamic signaling peptide GHRH. Like sermorelin and other GHRH analogs, Mod GRF (1-29) is capable of releasing large amounts of growth hormone (GH) from the human pituitary, even in individuals who have impaired release, provided pituitary somatotroph function is intact.
Unlike sermorelin (a forty-four amino acid peptide), Modified GRF (1-29) has the final fifteen acids removed; this improved stability without affecting function. Additionally, the final four remaining aminos are substituted (relative to endogenous GHRH and sermorelin) to further improve stability and increasing the amount of GH release since more of the peptide reaches the pituitary. Soule et al (1994) write that "D-Ala2 substitution contributes to the enhancement of biological activity by reducing metabolic clearance”.
In most individuals of all ages, large amounts of GH reside in the pituitary in specialized cells called somatotrophs. Somatotrophs produce and store GH, but the release of GH is determined by the hypothalamic signaling hormone GHRH and the gut peptide ghrelin. GHRH is also called GRF, and partial analogs of ghrelin are “GHRPs” (growth hormone releasing peptides). Collectively, these peptides and their synthetic counterparts are referred to as “growth hormone secretagogues.”
Endogenous GH plays a key role in development and growth, and is thought to contribute to increased recovery in younger athletes, but with aging pituitary GH release diminishes. Stress, illness, and lack of sleep can also decrease GH release.
Compared to exogenous GH (recombinant human growth hormone, or rHGH), growth hormone secretagogues more closely mimic physiology, with high peaks and low troughs of GH circulating for 2-3 hours as opposed to 10-12 or longer. As a result, insulin sensitivity is preserved and side-effects of rHGH such as fluid retention are typically avoided, as is receptor downregulation (a mechanism of tolerance and unwanted side-effects).
GHRH and its analogs, such as Mod GRF (1-29), amplify release of GH from active pituitary somatotrophs, but do not overcome the inhibitory hormone somatostatin. Somatostatin provides a braking system for GH releasing peptides and somatotrophs, while ghrelin and its GHRP analogs effectively release the brakes. As a result, GHRH is not as effective alone compared to when combined with a ghrelin analog (GHRP), unless an endogenous (unpredictable) ghrelin spike is occurring, enhancing somatotroph participation.
Despite the potential limitations of monotherapy Mod GRF (1-29), Lanes and Carillo (1994) concluded that "GHRH (1-29) at the dose and schedule used is generally effective in the treatment of GH deficiency." 
The increase in efficacy from combining the two secretagogues is a two-way street. Pandya et al (1998) find that "GHRH is necessary for most of the GH response to GHRP-6 in humans."  Massoud et al write that "Hexarelin and GHRH-(1-29)-NH2 are synergistic" 
 Soule S, King JA, Millar RP. Incorporation of D-Ala2 in growth hormone-releasing hormone-(1-29)-NH2 increases the half-life and decreases metabolic clearance in normal men. J Clin Endocrinol Metab. 1994 Oct;79(4):1208-11.
 J Izdebski, J Pinski, JE Horvath, G Halmos, K Groot and AV Schally. Synthesis and Biological Evaluation of Superactive Agonists of Growth Hormone-Releasing Hormone. Proceedings of the National Academy of Sciences, Vol 92, 4872-4876.
Lanes R, Carrillo E. Long-term therapy with a single daily subcutaneous dose of growth hormone releasing hormone (1-29) in prepubertal growth hormone deficient children. J Pediatr Endocrinol. 1994 Oct-Dec;7(4):303-8.
Pandya N, DeMott-Friberg R, Bowers CY, Barkan AL, Jaffe CA. Growth hormone (GH)-releasing peptide-6 requires endogenous hypothalamic GH-releasing hormone for maximal GH stimulation. J Clin Endocrinol Metab. 1998 Apr;83(4):1186-9.
Massoud AF, Hindmarsh PC, Matthews DR, Brook. The effect of repeated administration of hexarelin, a growth hormone releasing peptide, and growth hormone releasing hormone on growth hormone responsivity. Clin Endocrinol (Oxf). 1996 May;44(5):555-62.